**Core Concept**
Frame shift mutations are a type of genetic mutation that occurs due to the insertion or deletion of nucleotides in a DNA sequence that is not divisible by three. This results in a shift in the reading frame of the genetic code, leading to the synthesis of a completely different amino acid sequence.

**Why the Correct Answer is Right**
The correct answer is B. 3 because frame shift mutations occur when the number of inserted or deleted nucleotides is not a multiple of 3. This is because the genetic code is read in triplets of nucleotides (codons), and a frame shift mutation would disrupt this reading frame. For example, if three nucleotides are inserted, the reading frame would remain intact, but if one or two nucleotides are inserted, the reading frame would be disrupted. This is why frame shift mutations typically occur in multiples of 1 or 2, but not 3.

**Why Each Wrong Option is Incorrect**
**Option A:** A frame shift mutation could occur in multiples of 2, such as the insertion or deletion of two nucleotides, which would disrupt the reading frame.

**Option C:** A frame shift mutation could occur in multiples of 4, but this is not the correct answer because the question asks for the number that frame shift mutations DON'T occur in multiples of.

**Option D:** A frame shift mutation could occur in multiples of 5, but this is not relevant to the genetic code, which is read in triplets of nucleotides.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that frame shift mutations are often caused by errors during DNA replication or repair, and can lead to significant changes in the amino acid sequence of a protein, potentially resulting in disease.

**✓ Correct Answer: B. 3**