Final answer:
The complementary anticodon to the mRNA codon UCA is AGU. The letter options provided (A-D) in the question do not include the correct anticodon, as RNA base pairing rules must be considered.
Step-by-step explanation:
If an mRNA codon reads UCA, its complementary anticodon will be AGU. This is because during translation, the anticodons on the tRNA match up with the codons on the mRNA strand by the base pairing rules, but in RNA, adenine (A) pairs with uracil (U) and cytosine (C) pairs with guanine (G). So, if the mRNA codon is UCA, the tRNA anticodon that pairs with it would be AGU, where U pairs with A, C pairs with G, and A pairs with U.
The correct answer to this question is therefore AGU, which is notably absent from the multiple choice options provided. This is an important reminder to consider RNA base pairing rules, where uracil takes the place of thymine found in DNA.