A leukocyte is responsible for:

bringing oxygen to the other body cells
fighting disease by phagocytosis or antibody production
causing muscle contraction
conducting nervous impulses throughout the body

The second one is correct

Explanation:

A leukocyte is responsible for the immune system

nucleic acid molecules are incredibly complex, containing the code that guarantees the four different nucleotide units comprise dna, the nucleic acid of interest to the genealogist. and one of four nitrogen-containing molecules called nucleotides