What is the importance of codons differing in the last nitrogenous base (ex. cuu, cuc both coding for leucine) but still being able to code for the same amino acid?

This characteristic is called redundancy.

Redundancy is one of the characteristics of the genetic code, meaning that there may be several triplets (or codons) that code for the same amino acid. And these codons differ in the third nucleotide of the codon.

This characteristic has an interest in the conservation of protein sequences in case of mutations (we will have silent mutations that will not change the amino acid sequence).

It also has another interest that is to fill all possible combinations of codons: there are 64 different codons coding for 20 amino acids.

b would be the right answer

true, i don't really know what you're asking but from the info you provided im guessing the answer is true.