Explanation:

A single cell that does not function as a full-fledged organism refers to a cell that cannot survive or carry out all necessary functions on its own. It relies on other cells or a larger organism for survival and support. In the given options, the correct answer is option 'A' - White blood cells (WBC).

Here is an explanation of each option:

a) White blood cells (WBC):
White blood cells are an essential part of the immune system and are responsible for fighting off infections and diseases. However, they are not fully functioning organisms on their own. They are a part of the circulatory system and are produced in the bone marrow. They are constantly circulating throughout the body, seeking out and destroying pathogens. Without the support of other cells and organs in the body, white blood cells cannot survive independently.

b) Amoeba:
Amoeba is a single-celled organism that belongs to the group of protists. It is capable of carrying out all necessary life functions on its own, such as feeding, moving, and reproducing. Amoebas are considered full-fledged organisms because they can live independently.

c) WBC and Amoeba:
This option includes both white blood cells and amoebas. As explained earlier, amoebas can function as full-fledged organisms on their own. However, white blood cells cannot survive or carry out all necessary functions independently, so they do not meet the criteria of a single cell that does not function as a full-fledged organism.

d) Paramecium:
Paramecium is another type of single-celled organism that belongs to the group of protists. Like amoebas, paramecia are capable of carrying out all necessary life functions on their own. They can move, feed, and reproduce independently, making them full-fledged organisms.

Therefore, the correct answer is option 'A' - White blood cells (WBC), as they are single cells that rely on other cells and organs for survival and do not function as full-fledged organisms.