You are on a field trip and are recording the amount of mass in a volume of water. what are you measuring?

the concentration of bacteria in seawater
the concentration of dissolved oxygen in seawater
the concentration of salt in seawater
the density of seawater

When we record the amount of mass in a volume of water we are measuring the density of sea water.

The density of seawater

Explanation:

Density is defined as the amount of mass contained in unit volume of water. It is given by the equation D=mass/volume

The unit of the physical quantity density is .

Density is crucial in determining buoyancy. Buoyant force exerted by a denser liquid will be greater than the buoyant force exerted by a less denser liquid. Relative density is a dimensionless quantity that helps in understanding the density of  a liquid by comparing it with a standard density.

Relative density of a liquid is the ratio of density of the liquid to the density of water which is taken as the standard.

i think the answer is a.keratin is the pigment responsible for skin color.