If a tRNA molecule has an anticodon which reads AGGCCG, what was the codon on the mRNA molecule? TCCGGA
UCCGGC
UCCGGU
CTTAAG

The codon is the sequence of nucleotides that carry genetic information that determines which amino acid should be added to the building protein. Nucleotides in this sequence pair with nucleotides from the tRNA  anticodon. In the exposed example, the correct codon pairing the given anticodon is is UCCGGC.

Before expplanation, let us review some theoretical frame to answer the question.

During protein synthesis, the ribosome reads mRNA in the 5´ to 3´ direction, while tRNA adds the correct amino acids to build the polypeptide, according to the codons readen in the mRNA chain. A codon is a short sequence of three nucleotides that store the genetic information for the aminoacids´ assembly. Each of the codons represents one of the 20 amino acids used to build the protein. Each tRNA has two important sites. One of them that couples with the codon of the mRNA molecule, named anticodon. The other site couples with an amino acid. tRNA allows amino acids to align according to the nucleotidic sequence in the mRNA molecule.  

Once the new amino acid joins the growing protein, the binding between the amino acid and the tRNA breaks, releasing the molecule, which is now available to join another amino acid and repeat the cycle.  

• Anticodon: Sequence of bases that pairs with the mRNA codon.  

• Amino Acid: the amino acid that is codified by the mRNA codon.  

Remeber that:

In the exposed example, the tRNA anticodon is AGGCCG, so we need to find its complement.

We know that Adenine pairs Uracyl, Guanine pairs Cytosine, and Cytosine pairs Guanine. So,

Anticodon → AGGCCG

     Codon → UCCGGC    

Related link:

a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.