Because there are 64 possible codons and only 20 amino acids:
a. many mutations have no effect on phenotype.
b. every mutation has some effect on phenotype.
c. each trna molecule contains two extra codons.
d. each trna molecule contains two extra amino acids.

Option A

Explanation:

There are 64 possible codons, out of which 61 codons code for particular amino acid and three codons are stop codons. As there are 61 codons for 20 amino acids, some of the amino acids are coded by more than one codons.

Due to this many mutations cannot show their phenotypic effect. For example, if an original codon UUA  gets converted into UUG due to a point mutation, there would be no noticeable change seen in the phenotype of the organism because both of the codons code for the same amino acid, leucine.

a tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.

the answer is: reproductively

i think i spelled it wrong lol

anyways hope i

-lavira: )