At the cellular level indicate that the living organisms are made up of the smallest building blocks called cell. It refer to unicellular organisms i.e organism with one cell.
At the cellular level, membranes; for the whole organism, the skin is Reproduction because a single parent cell divides into two cells (offsprings), two divides into four cells and so on. It is called cell division and cell reproduction because a single cell skin divides to produce many cells.
The best way to find the correct answer for a sentence completion would be : find that part of the text and look for the answer. With this , you can find the answer in a more efficient way and in a shorter amount of time
