Ascientist who is researching marshes observes that marsh grass, spartina bakeri, grows to different heights at different locations in the marsh. if the scientist wants to find the reason for this difference, what is the correct first step?

As per correct scientific methodolgy, the first step that scientist needs to take is to formulate a hypothesis. Because he already has went through first two steps of scientific method, 1: Observation. 2: Question. So he should proceed with formulating a hypothesis.

A hypothesis is basically a preposition which scientists formulates based on the incomplete information he has. For example: In this case, the scientist can assume that  marsh grass, Spartina bakeri, grows taller on east side than on west side because east side contain more nutrient enriched marshes.

This is not a fact but just a preposition that he will prove or disapprove when he will conduct the whole experiment and find the facts.

So a this stage he needs to formulate a hypothesis based on little knowledge he has.

Hope it helps!

the answer to your question is:

c. complementary base-pairing