Answer: D. The main source of energy that a cell uses to carry out its daily actions
Let us review our answer choices:
A. "Not used by cells". Even the basic building blocks of life need ATP energy in order to function.
B. "Only found in the cytoplasm of cells". While ATP can be stored in the cytoplasm, it is not only found in the cytoplasm and in fact originates within the mitochondria also known as the powerhouse of the cell.
C. "A part of the cell that digests other cells". This is simply incorrect, ATP's function does not involve the digesting of other cells, and rather aids in the production of internal cellular processes.
D. "The main source of energy that a cell uses to carry out its daily actions". The energy the cell needs to carry out its "daily actions" provides itself in the form of ATP.
After further deduction, we are left with D. as our correct answer.