A cell begins to produce a new type of protein. This is most likely due to an alteration of the

nucleotide sequence of its template DNA strand

Explanation:

If a cell begins to produce a new type of protein, it is most likely due to an alteration of the nucleotide sequence of the template DNA strand of its genome.

The alteration of the nucleotide sequence (mutation) of the template DNA strand leads to a corresponding change in the codons of the mRNA transcribed from such DNA. A change in the mRNA codon leads to a change in the amino acid translated from the mRNA codons.

Amino acids are monomers of proteins and as such, a new protein results from altered DNA sequence.