There are a few ways to get to the answer of the splice error:
The mutation caused "skipping" of an exon. Splicing is the process where introns are spliced out and exons are spliced together before exporting the mRNA. Retained introns or skipping of an exon is likely a result of an error in splicing.
The mutation itsself is GT to AT. In RNA this would be GU to AU. GU is a known component of the 5' splice site (FA2020p42), so this also points to splice site mutation.
None of the other mutations adequately describe why an exon would be skipped. You could reason a nonsense mutation (a mutation that introduces a stop codon) could "skip" the last exon or so because it trunctuates the protein. However, the mutation does not fit any that introduce a stop codon (the stop codons are UAA, UGA, and UAG... none of these have AU in them!)
Other info: this boy has phenylketonuria, though multiple mutations can lead to this so it would not help in answering this question.