Hey Haewon! =)

Great question! For a clearer explanation, let's take a look at what Martone and Niklas say

Martone:

Lines 67-73

Martone is "convinced that red and green algae descend from one endosymbiotic event, which is about a eukaryote cell gaining the ability to photosynthesize (produce food on its own).

Niklas:

Lines 74-75

Niklas agrees with Martone about the evidence for lignin, which is the molecular pathways.

However...

Lines 76-78

Niklas believes red and green algae evolved from separate endosymbiotic events, not from one event.

So Niklas's counterargument is about the endosymbiotic events (not about the molecular pathways since he agrees with Martone about that). That is why (A) is not correct.

(C) 

The evolutionary development of red and green algae is related to the endosymbiotic events since they are part of the algae's evolution. Hence, the evolutionary development "diverging" (splitting) before the ability to photosynthesize does relate to Niklas believing the algae evolved from separate endosymbiotic events since the endosymbiotic event is what allowed the algae to photosynthesize.

This is a difficult question since there are several dots that have to be connected. Hope the explanation makes sense! The simple answer is that you can eliminate (A) since "major molecular pathways" don't relate to Niklas's counter, which is what the question asks you about. =)

Keep Moving Forward!