A cell

a. can be composed of many types of tissues.

b. is found only in plants and animals.

c. is the smallest entity studied by biologists.

d. may be a distinct entity or a building block of a more complex organism.

d. may be a distinct entity or a building block of a more complex organism.

Explanation:

A cell is a structure that is able to live freely as it can perform all the vital processes that are required for life. Unicellular organisms have a single cell in their body. Therefore, the single cells may exist independently in the form of unicellular organisms such as Amoeba, Chlamydomonas. Similarly, many cells together form the multicellular bodies of complex organisms. These cells may be loosely packed as in sponges or may show tissue / organ or organ system level of organization.