**Core Concept:**
A frameshift mutation is a type of genetic mutation that occurs when a single nucleotide insertion or deletion results in a change in the reading frame of a protein-coding sequence in DNA or RNA. This can lead to the production of a non-functional protein or a protein with a completely different function.

**Why the Correct Answer is Right:**
The correct answer, **D. Insertion**, causes a frameshift mutation because an insertion of a nucleotide into the DNA sequence shifts the reading frame, altering the amino acid sequence encoded by the affected gene. This can disrupt the normal structure and function of the protein product.

**Why Each Wrong Option is Incorrect:**
A. Deletion (Option B) does not cause a frameshift mutation because it removes nucleotides rather than adding them, thus maintaining the reading frame and potentially resulting in a protein with altered function but not necessarily a non-functional protein.

C. Point mutation (Option C) involves a single nucleotide substitution rather than insertion or deletion, and therefore does not cause a frameshift mutation.

**Clinical Pearl / High-Yield Fact:**
High-Yield Fact: Frameshift mutations are associated with several genetic disorders, including cystic fibrosis, sickle cell disease, and certain types of cancer. Understanding the concept of frameshift mutations is essential for interpreting genetic data and diagnosing genetic disorders.

**Correct Answer:**
Correct Answer: Insertion (Option D) causes a frameshift mutation due to the alteration of the reading frame in DNA or RNA, resulting in a non-functional protein or a protein with a different function.