In a 125 amino acid sequence, if 25${}^t$${}^h$ amino acid is mutated to UAA, then

The correct option is C A polypeptide of 24 amino acids is formed.
UAA is a stop codon. It does not code for any amino acid and, therefore, called nonsense codon. Hence, the polypeptide synthesis is terminated and a polypeptide of 24 amino acids is formed. The polypeptide chain of 124 amino acids is formed only when the 125${}^t$${}^h$ codon works as a stop codon.

So, the correct answer is 'A polypeptide of 24 amino acids is formed'.