Correct answer: Cells are the structural and functional units and are the building block of organisms. Hence, the correct answer is option (c).

Reason for correct answer:

The biological organization of organisms starts from atoms that are the smallest unit. Many atoms constitute molecules and many molecules constitute cells. All living organisms are made of cells including unicellular or multicellular. Several cells constitute a tissue. Thus, for all living organisms, cells are the structural building block.

Option (c) is given as “are the building blocks of living organisms”.

Cells are the building blocks of living organisms and they also make a tissue. Hence, the correct answer is option (c).

Reason for incorrect answer:

Option (a) is given as, “are not found among the bacteria”.

Bacteria are unicellular organisms made of a single cell. Hence, option a. is incorrect.

Option (b) is given as, “always have nuclei”.

Red blood cells seen in human beings lack a nucleus. Several microorganisms do not have nucleus, instead they have a nucleoid. Hence, option b. is incorrect.

Option (d) is given as, “are made up of tissues”.

Several cells that perform a common function constitute a tissue. Tissues do not constitute the cells. Hence, option d. is incorrect.

Option (e) is given as, “a and b are true”.

Bacteria are unicellular organisms made of a single cell. Red blood cells seen in human beings lack a nucleus. Several microorganisms do not have nucleus, instead they have a nucleiod. Options (a) and (b) are false. Hence, option (e) is incorrect.

Hence, the options (a), (b), (d), and (e) are incorrect.