Nucleic acids are composed of: a: a phosphate group, a pentose sugar, a nitrogen base
b: a phosphate group, a pentose sugar, a carboxylic acid
c: a pentose sugar, a nitrogen base, an R group
d: a nitrogen base, an amino acid group, a carboxylic acid

a phosphate group, a pentose sugar, a nitrogen base

Explanation:

nucleic acid is a polymeric macromolecule made up of repeated units of monomeric 'nucleotides' composed of a nitrogenous heterocyclic base which is either a purine or a pyrimidine, a pentose (five carbon) sugar (either ribose or 2′-deoxyribose), and one to three phosphate groups.

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.