This isn’t the purpose, because (A) ignores the context. The rest of the passage is about secondary substances and the development of secondary substances. So in describing primary substances and how they’re different from secondary substances, the author’s purpose relates to secondary substances. The author isn’t merely trying to teach us how plants grow and absorb nutrients. That purpose wouldn’t have any connection to the rest of the passage and its focus on secondary substances.

This best captures the purpose. The author tells us what primary substances do in order to help us understand what secondary substances don’t do. This is part of an explanation of secondary substances.

The ways that insects have affected plant evolution aren’t brought up in P1 and don’t relate to the description of primary substances. So (C) can’t be the purpose of mentioning primary substances.

Although the author mentions several different primary substances, the author never characterizes these as showing a great diversity. Note that the author does say that secondary substances are a “diverse and multitudinous array of chemicals,” but she doesn’t say this with respect to primary substances. So (D) isn’t the purpose of the discussion of primary substances.

Nothing in P1 provides evidence of plants’ adaptation to insects. The evidence comes in P2.