Q:

Is "bananas is" possible by any chance?

My brother is a highschooler. His mid-term English-language exam is early next month. His teacher gave him an assignment in preperation for the exam. One of the questions given is:

(Bears - bananas - broccoli) is sweet fruit.

I called the teacher telling him that none of the choices make a correct sentence: Broccoli is a vegetable, bears are animals, and bananas should be "banana" to be in agreement with the singular verb is. However, he keeps arguing bananas is the correct choice. 
Edit
His argument is that "bananas" refers to the substance that makes up the fruit, rather than the units, pretty much as chicken refers to the substance, or flesh, in " I like chicken" - not chickens. So he says bananas is equal to chicken in this example.

A:

If you use "banana" as a measured recipe ingredient, it's an uncountable, singular noun:

Banana can be used as a substitute for egg, as it binds the ingredients together.
The recipe says to add a cup of banana to the cake mix.

Usually the recipe will call for a certain number of bananas (rather than an amount) since once you peel a banana you need to use it, and it's much easier to measure individual bananas than portions of banana:

This cake recipe says we need three bananas.  Are there any in the pantry?

It's pretty much the same for all fruit: apple/apples, peach/peaches, and so on. 
That being said, it's not natural to say, "Banana is a sweet fruit".  You could say either:

Bananas are (a) sweet fruit.

or

A/The banana is a sweet fruit.

Btw, I think the teacher meant to write "pears" instead of "bears", which are otherwise not normally confused with one another.
(Edit) Apparently I misunderstood, and it's a "which of these three is correct" type of question.  So the question could have been better phrased, "Which of the following is a sweet fruit: bears, broccoli, or bananas?" 

A:

His argument is that "bananas" refers to the substance that makes up the fruit, rather than the units, pretty much as chicken refers to the substance, or flesh, in "I like chicken" - not chickens. So he says bananas is equal to chicken in this example.

I want to debunk his argument. He agrees that we'd say, "I like chicken" (not chickens) – but that is the singular form of the word. So I wonder why he thinks we'd switch to the plural, and say the very ungrammatical, "Bananas is  sweet fruit." He is dishing out bad guidance here. 
I agree with Andrew; these are correct ways to say this: 

Bananas are a sweet fruit.
A banana is a sweet fruit.

This one is also valid: 

The banana is a sweet fruit. 

(Here, the definite article the indicates we are referring to all bananas, not one particular banana.)
One could even say: 

Bananas are sweet fruits. 
Banana is sweet fruit. 

I think both of those are less common, but they are both grammatical. 
But the one he is using: 

Bananas is sweet fruit. 

should be avoided in all cases. 
Going back to his chicken argument, I can say either one of these:

Chicken is healthy meat. 
Banana is sweet fruit. 

But I would not say, "Chickens is healthy meat," nor would I say, "Bananas is sweet fruit."

A:

It is possible, though slightly torturous, to come up with a scenario where this works.  For example, if you create a dessert dish of semi-smashed banana chunks, maybe cooked a little, you might refer to this dish as "bananas."  Akin to calling a bowl of sliced cling peaches "peaches."
If you eat this as part of a meal, you might say "bananas is sweet fruit, but mashed potatoes is savory starch."  You're no longer referring to discrete countable banana items, but the count-less substance in the bowl.
For an English-language learner?  No, definitely it should be "bananas are sweet fruit" or "bananas are sweet fruits," or "banana is a sweet fruit."  For the "substance" situation that the teacher discussed, I think most native speakers would say "banana is sweet fruit."
You could also say "you calling me bananas is nuts." =)

