The following relationships are known: CGG specifies Arginine, GGC specifies Glycine and GCG specifies Alanine. Based on your knowledge about the genetic code, it would be most likely that CGA specifies:

Arginine

Explanation:

A specific genetic code specifies a particular amino acid. One amino acid can be encoded by more than one genetic code.

When a particular amino acid is encoded by more than one genetic code, all the genetic code for that amino acid mostly share common first two bases and differ from each other in the last base. This allows the same tRNA with particular anticodon to read all the possible genetic code for that amino acid and to serve as a carrier due to wobble base pairing.

Given that code "CGG" specifies arginine, the code "CGA" should also specify "arginine" since both differ with respect to the last base only.