All of the options except "b) Unicorn" are real world objects. A book, a pen, and a chair are all tangible objects that exist in the physical world.
a) Book
b) Unicorn
c) Pen
d) Chair
### Instruction:
Which of the following objects is a real world object?

### Input:
a) Book
b) Unicorn
c) Pen
d) Chair

### Response:
