Male behaviour during breeding season
a) stronger aggressive behaviour only during egg laying
c) stronger aggression when egg is first laid than during nest building
d) greater aggression during hatching eggs than during nest building

These all occur after the stage which determines fitness. Egg laying (a and c) and Egg hatching (d) occur after the male has won the right to mate.

b) stronger aggressive behaviour during nest building rather than
during hatching of eggs

Assuming the behavior increases mating success and fitness, the answer is B because this is the only strategy that attracts more females and gets rid of rival males before it is too late (i.e. before the female mates). This is also assuming that the female can/will only mate once.
In chronological order the answers run &quot;B,C/A,D&quot; (C &amp; A are really one and the same in my eyes). Mating occurs between B and C.