What is apoptosis?
a. a pathological condition observed only in damaged or diseased organisms
b. an experimental technique used to kill specific cells
c. a developmental mechanism unique to the roundworm c. elegans
d. programmed cell death that is required for normal development

The correct answer is option D.

Explanation:

Apoptosis is a way of programmed cell death that takes place in all the multicellular living beings. This type of the cell programmed leads to the characteristic change and the death of the cells due to biochemical events that took place in cell.

These process is important in multicellular organism to grow normally by change and death of aged or damaged cells. The change that take place in this process are nuclear fragmentation, cell shrinkage, chromatin condensation and other cellular changes.

Thus, the correct answer is option D.

substitution is the answer in this case