Question

about $\frac{2}{3}$ of the mass of this cell is
a air.
b ribosomes.
c proteins.
d water.

Explanation:

Water is the most abundant component of cells, typically making up about two - thirds of a cell's mass. Air is not a major mass - contributing component within the cell. Ribosomes and proteins, while important cellular components, do not constitute about two - thirds of the cell's mass.

Answer:

D. water