Where should an object be placed in front of a convex lens to get a real image of the size of the object?

Q. 3 :- Where should an object be placed in front of a convex lens to get a real image of the size of the object? 
(a) At the principal focus of the lens
(b) At twice the focal length
(c) At infinity
(d) Between the optical centre of the lens and its principal focus.

 

Answer:-

(c) At infinity