Anormal polypeptide includes the following sequence of amino acids(note: each amino acid is named by a three-letter abbreviation.)

iso-leu-pro-val-his-ser-thr-met

after a mutation, the amino acid sequence becomes the following:

iso-leu-pro-val-tyr-arg-iso-gly

which of the following changes to a nitrogenous base in mrna is most likely to have occured?

a. a cytosine(c) was replaced with uracil(u).
b. a uracil(u) was replaced with a thymine(t)
c. an adenine( a) was removed and not replaced
d. the codon acu was removed and not replaced