Threonine is an aminoacid that can be coded with these 4 codons: ACT, ACC, ACG, ACA.
Arginine can be coded with these codons: CGT. CGC, CGG, CGA, AGA, AGG
The answer is ACA was transcribed instead of AGA.
From those options this is the only one that works.
AGA would code for Arg but due to the error, it was transcribed as ACA and became Thr.