Question

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

Explanation:

Cells are composed of various substances, and water is the most abundant component, making up about two - thirds of a cell's mass. Air is not a major component of cell mass. Ribosomes and proteins, while important cellular components, do not constitute about two - thirds of the cell mass.

Answer:

D. water