**Core Concept**
Telomerase is a reverse transcriptase enzyme that extends telomeres, the repetitive nucleotide sequences at the ends of chromosomes, thereby maintaining their length. This enzyme plays a crucial role in preventing telomere shortening, which can lead to cellular aging and senescence.

**Why the Correct Answer is Right**
Telomerase is predominantly found in stem cells, germ cells, and certain types of cancer cells, where it is necessary for maintaining telomere length and promoting cell division. In these cells, telomerase is expressed to counteract telomere shortening, thereby allowing for continuous cell division without the accumulation of senescence. This is particularly important in stem cells, where telomerase activity helps maintain their self-renewal capacity.

**Why Each Wrong Option is Incorrect**
**Option A:** While some cancer cells do express telomerase, not all cancer cells exhibit telomerase activity.
**Option B:** Most somatic cells, including fibroblasts, do not express telomerase under normal conditions.
**Option C:** While some cells in certain tissues may express low levels of telomerase, this is not a defining characteristic of these cells.

**Clinical Pearl / High-Yield Fact**
Telomere shortening is a hallmark of aging and is associated with various age-related diseases, including cardiovascular disease and osteoporosis.

**Correct Answer:** B. Fibroblasts.