The correct option is
D S-P-R-Q
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.
S is first in the sequence: The first line says 'for a moment'. S continues by telling that us for a moment the subject forgets.
S is followed by P: P continues by telling us what the subject forgets. It says that the subject forgets that he/she is terribly old.
P is followed by R: R adds to P. It says that in addition to that subject also forgets that it is a very long time ago.
R is followed by Q: Q tells us what is a very long time ago. It says that it is a very long time ago since the subject was a child.
Finally, the correct sentence is 'For a moment I forget that I am terribly old and that it is a very long time ago since I was a child'.
Thus, the correct sequence is S P R Q. Hence, D is the correct option.