The correct option is
A Q-P-S-R
The first statement in the sequence should be the one that best connects with the first line. It must be a continuation or extension of the first line.
Q is first in the sequence: Q is the only phrase that connects with the first line. Thus, the line becomes 'For us to complete this work'.
Q is followed by P: P tells us when they have to complete the work. It says that for them to complete the work before Monday was something.
P is followed by S: S tells us what that something is. It says that to complete this work before Monday is very important.
S is followed by R: R adds to S' point. It says that to complete the work is important and it should not be delayed.
Finally, the corrected sentence is 'For us to complete this work before this Monday is very important and it should not be delayed'.
Thus, the correct sequence is Q P S R. Hence, B is the correct option.