D. Arg is replaced by His
The code is read in groups of 3 (triplets), so the triplet that is changed is:
GCA -> GTA
From the table, we know that GCA results in the amino acid ARG.
However the C changed to a T in the mutation so the trip is now GTA which codes for His instead according to the table. Therefore Arg is replaced by His. It is option D.
Hope this helped!