Q:

Conditionally add items to a drop-down list

I have two columns in Google Sheets: one is the item Name and the other is an indicator of whether it should be included in a drop-down:

In this example I want the drop-down to include Item B and Item C and if I were to change the flags in the Include column I would want my drop-down either to be expanded or constrained as specified.  
How should I create a drop-down which will populate based on the Include column?

A:

Assuming Name is in A1 of Sheet1, then perhaps in C2: 
=if(B2="Yes",A2,"") 

copied down to suit and your Validation..., Criteria: List from a range:  
=Sheet1!C:C

