Which of the following correctly identifies letter d on the diagram above?

"Which of the following..." indicates that you had a list of answer choices.  There are a couple of different names for D, and without those choices, it is impossible to know exactly which of the possible names is correct.  It could be brainstem, it could also be the medulla or the medulla oblongata.  

the generally accepted parts of modern cell theory include: all known living things are made up of one or more cells. all living cells arise from pre-existing cells by division. the cell is the fundamental unit of structure and function in all living organisms.

several tissues working together form an organ, and several organs together make up an organisms.

the group of cell that carries out special jobs are tissue.