If a portion of a messenger RNA molecule contains the base sequence A-A-U, then the corresponding anti-codon on the tRNA base sequence is

U-U-A

Explanation:

A mRNA molecule is formed as a result of transcription, which then proceeds to the ribosomes in the cytoplasm to undergo translation. Translation is the process whereby an amino acid sequence (protein) is synthesized using the information in the mRNA molecule/sequence. The mRNA sequence is read in a group of three nucleotide bases called CODON. Each codon specifies an amino acid.

This codon is read by another RNA molecule in the ribosome called tRNA (transfer RNA). tRNA contains another set of three nucleotides called ANTICODON, which is complementary to the CODON and responsible for reading the codon in order to carry the corresponding amino acid to the growing sequence.

According to this question, a portion of a messenger RNA molecule contains the base sequence A-A-U, the complementary anticodon sequence of the tRNA would be U-U-A.