Answer:
d. hairpin
Step-by-step explanation:
A double-stranded base-paired structure formed by folding a single strand of DNA or RNA back upon itself like a hairpin. Base pairs are the unit of nitrogenous bases which are joined together by hydrogen bonds. They synthesize the double helix model which contribute the folded structure of both RNA and DNA. Sugar and phosphate is the backbone of DNA. DNA has four nitrogenous bases named adenine, thymine, guanine, and cytosine.