Final answer:
The correct answer is 'd. Hairpin loop,' which is a structure formed by a single-stranded DNA or RNA molecule that folds back on itself to form intramolecular hydrogen bonds between complementary nucleotides.
Step-by-step explanation:
A structure formed by a single-stranded DNA or RNA molecule that has complementary sequences is known as a hairpin loop. This structure occurs when a strand of DNA or RNA folds back on itself and forms intramolecular hydrogen bonds between complementary nucleotides, creating a loop that resembles the shape of a hairpin.
In the context of DNA, the double helix structure is characterized by antiparallel strands that are connected through hydrogen bonding, with adenine pairing with thymine, and guanine pairing with cytosine. This forms the iconic spiral staircase or twisted ladder appearance discussed in many descriptions of DNA.
However, a hairpin loop is distinct as it pertains to single-stranded molecules. Because we are discussing a structure formed by single-stranded molecules that fold upon themselves, the correct answer to the question is 'd. Hairpin loop'.