Why do some codons code for the same amino acid as another codon?
a. it is due to mutations.
b. there are only 20 amino acids and 64 possible combinations
c. each codon is unique and they all code for different amino acids.

B. There are only 20 amino acids and 64 possible combinations

Explanation:

The genetic code is degenerate this means that more than one codon codes for a single type of amino acid.This degeneracy occurs because there are 64 possible triplets of the bases that exist however there are only 20 amino acids known to encoded by the genetic code. Out of this 64 possible triplet codons, 61 triplets are responsible for coding the amino acid whereas three of them form a stop codon that helps terminate the translation.