Final answer:
A hairpin loop in DNA is mostly composed of single-stranded DNA, which folds back on itself and forms a loop. Complementary base pairing is involved in the formation of hairpin loops, but it is not the primary component.
Step-by-step explanation:
The main component of a hairpin loop in DNA is single-stranded DNA. Hairpin loops are formed when a single DNA strand folds back on itself and forms a loop. This can occur when a sequence of complementary bases within the same strand is present, causing the strand to fold and form a loop. Complementary base pairing is involved in the formation of hairpin loops, but it is not the primary component.