While working with cultured mouse cells, a researcher unknowingly treated the cells with a mutagen that causes the deletion or insertion of individual nucleotides in DNA. Subsequently, she isolated and cultured a single cell from this group. She noticed that the progeny of this cell were not producing a certain protein and that this affected their survival. The mutation would be most harmful to the cells if it resulted in .

The correct answer is - a single nucleotide insertion near the start of the coding sequence.

Explanation:

Insertion mutations are the mutation that takes place when there is an extra nucleotide is added to the DNA strand during replication and causing the particular protein translation or inhibits the translation if present at the start codon.

It did not allows the translation in this sequence, Deletion, and insertion both are a dangerous mutation that causes the production of unwanted protein or not producing the protein and if this protein is essential for surviving it will lead to cell death.