Final answer:
An amino acid is the building block of proteins that is added to the growing protein chain in a ribosome during translation, based on the codon sequence encoded in mRNA. Therefore, the correct option is C.
Step-by-step explanation:
The building block of proteins that are added to the protein chain within a ribosome based upon the information encoded in the mRNA base sequence is an amino acid. During the process of translation, codons on the mRNA are read by the ribosome and matched with transfer RNA (tRNA) that carry the appropriate amino acids. Each group of three nucleotide bases in mRNA constitutes a codon, which specifies a particular amino acid. For instance, the codon GGU codes for the amino acid glycine. The tRNA has a complementary region known as an anticodon, which pairs with the codon, allowing the tRNA to deliver the correct amino acid to the growing polypeptide chain. This sequence of events ensures that proteins are assembled accurately according to the genetic information carried by the mRNA.