Ans.
Fragile X syndrome is defined as a genetic disease, results in cognitive impairment and mental problems.
It is caused by a mutation in FMR1 gene that causes an increase in the CGG trinucleotide units in the gene. This is an example of insertion mutation as it involves addition of nucleotide sequences to the gene.
The FMR1 gene codes for a protein, which is necessary for normal cognitive functions and defect in this protein, due to mutated gene leads to development of disorder.
Thus, the correct answer is option D). 'insertion.'
