Answer: A. Phosphodiester bond.
Explanation: "A phosphodiester bond occurs when exactly two of the hydroxyl groups in phosphoric acid react with hydroxyl groups on other molecules to form two ester bonds. Phosphodiester bonds are central to all life on Earth as they make up the backbone of the strands of nucleic acid."
"The nucleotides in DNA and RNA have three components: a sugar group, a nitrogenous base, and a phosphate group. Adjacent nucleotides in a nucleic acid are joined by a phosphodiester bond."