Answer:
deletion or insertion of nucleotide from or in any coding sequence of the DNA
Explanation:
The mutation would be most harmful to the cells if it resulted in the deletion or insertion of nucleotide from or in any coding sequence of the DNA.
Deletion or insertion of a nucleotide from/into the coding sequence of DNA results in a frameshift mutation away from the original protein. Deletion/insertion of a nucleotide from/to the coding sequence of a DNA can cause a totally different protein to be synthesized from such DNA or, worse still, results in a nonfunctional protein. Both results could be grossly harmful to the cells.