Which is an example of making a quantitative observation?

measuring the mass of metal used in a reaction

recording the color of a solid formed in a reaction

describing a liquid as having a slippery feel

rating an odor on how nice it smells

measuring the mass of metal used in a reaction

Explanation:

Quantitative observation is taking any kind of measurement using a tool like measuring the volume of water or weighing the mass of a metal.  

Recording the color of a solid formed in a reaction is a qualitative observation. There is no metric involved but defining the color is a qualitative task.  

Describing a liquid as having a slippery feel is a qualitative observation.  

Rating an odor on how nice it smells is a qualitative observation.

c- cold periods in earth's history during which glaciers covered much of the surface.

explanation:

the atomic mass is the bottom number on the element.

-steel jelly