Question

1- He has been absent ........monday (preposition)
A since
B for
C from
D by

Answer 1

He has been absent since Monday.

A preposition or post position basically is a combination with a noun or pronoun, or more obviously with a noun phrase, also called its complement, or at times an object.

This always comes before its complement that is a post position takes in a position after its complement. 

Answer 2

Here is your answer,,,

He has been absent since Monday.