The correct option is
C S P R Q
The first statement in the sequence should be the one that best connects with S1. It should be a continuation or extension of S1.
S is first in the sequence: S1 starts by saying that salmon fish pushed themselves. S continues by saying that they pushed themselves to their limit.
S is followed by P: P tells us why they pushed. It says that they pushed themselves to their limit to return to their spawning grounds.
P is followed by R: R adds a condition after P. It says that although they pushed themselves to return, once they laid the eggs.
R is followed by Q: Q and S6 together tell us what happened after they laid eggs. It says that once they laid their eggs and fertilized them, they died.
Finally, the correct sentence is 'The salmon fish pushed themselves to their limits to return to their spawning grounds but once they laid their eggs and fertilized them they died'.
Thus, the correct sequence is S P R Q. Hence, C is the correct option.