a. Posphate and hydroxyl
Explanation:
Hydroxyl group on carbon No.5 of ribose or deoxyribose on one nucleotide will react with hydroxyl group on phosphate on another nucleotide. Phosphodiester bond is formed by condensation reaction which is called as esterification.
Enzymes involved are DNA polymerase and DNA ligase in DNA replication
and RNA polymerase and ligase in RNA transcription
DNA has deoxyribose
RNA has ribose