Which codon has dual functions?

The correct answer is Option (4) → AUG

Option (4) AUG — Correct. AUG has a dual function: it codes for the amino acid methionine, and it is also the start (initiator) codon where translation begins.

NCERT Extract:
Genetic Code – “… UUU would code for Phenylalanine (phe). Some exceptions to this rule have been found in mitochondrial codons, and in some protozoans. (v) AUG has dual functions. It codes for Methionine (met) , and it also act as initiator codon.”