The operation on a queue returns the last element in the queue, but does not remove the element from the queue
a.
front
b.
back
c.
pop
d.
push

 back

Explanation:

queue is the data structure which perform operation in particular order.  the order is first in first out (FIFO).

it means the element which comes first, remove first.

front: it is used to fetch the first element or oldest element in the queue.

back:  it is used to fetch the last element or newest element in the queue.  

pop: it is used to remove the first element or oldest element in the queue.

push: it is used to insert the element at the back in the queue.

Therefore, the back is the correct option.