Question

living things use fats to store energy, but fats also form important parts of
(this answer requires two words)
○ biological molecules
○ nucleic acids
○ carbon compounds
○ biological membranes

Explanation:

The question is about the role of fats in living things, specifically what important parts they form. Fats (lipids) are a major component of biological membranes (like cell membranes). The other options: biological molecules is too broad, nucleic acids are DNA/RNA, carbon compounds is too general. So the correct two - word answer related to the part fats form is "biological membranes".

Answer:

biological membranes