The telocentric chromosomes carry the centromere at one end . Their shape is similar to the letter i. Usually they are not seen in healthy humans, since they are unstable & arise by breakage near the centromere .

The correct answer is option 'B' - tolecentric.

Explanation:
To understand why a chromosome carrying the centromere at one of the ends is called tolecentric, let's break down the terms and their meanings:

1. Centromere: The centromere is a specialized region of a chromosome that plays a crucial role in cell division. It is responsible for the attachment of spindle fibers during mitosis and meiosis, ensuring the proper separation of chromosomes.

2. Tolecentric: Tolecentric refers to the position of the centromere on a chromosome. It is a term used to describe a chromosome where the centromere is located near one of the ends.

Now, let's explore the characteristics and structure of tolecentric chromosomes:

1. Centric vs. acentric chromosomes:
- A centric chromosome is one that possesses a centromere, thereby ensuring proper segregation during cell division.
- In contrast, an acentric chromosome lacks a centromere and cannot properly attach to spindle fibers, leading to improper segregation during cell division.

2. Metacentric vs. acrocentric chromosomes:
- Metacentric chromosomes have centromeres located near the center, resulting in two arms of equal length.
- Acrocentric chromosomes have centromeres located closer to one end, resulting in one long arm (q arm) and a short arm (p arm).

Now, let's focus on tolecentric chromosomes:

- A tolecentric chromosome is a specific type of acrocentric chromosome where the centromere is positioned very close to one end, making the short arm extremely short or almost non-existent.
- The term "tolecentric" is derived from the Greek word "tole" meaning "end" or "tip," indicating the location of the centromere.

To summarize:
- A chromosome carrying the centromere at one of the ends is called tolecentric.
- Tolecentric chromosomes are a type of acrocentric chromosome where the centromere is positioned very close to one end.
- The centromere's location in a tolecentric chromosome results in an extremely short or non-existent short arm.