Which letter corresponds to the monomer of nucleic acid ?

Monomer of nucleic acid is Nucleotide.

Nucleotide is made up of three components namely- Nitrogenous base, pentose suger and phosphate group.

Depending upon the nucleic acid,i.e RNA or DNA, nucleotide can be Ribonucleoside triphosphate (rNTP) and deoxyribonucleoside triphosphate repectively (dNTP).

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).

b. lithosphere, asthenosphere, mesosphere   is the answer most likely