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:

In chemistry we analyse any matter in two ways

a) Qualitative: For qualitative analysis we study quality of matter which includes

i) its chemical nature: example organic / inorganic/ cation /anion/ acid/ base / aromatic/ cyclic etc.

ii) its physical nature : example color, odor , viscosity. lubricity, density, temperature, pressure etc.

b) Quantitative: For quantitative analysis we study the quantity of matter.

For example

Mass or volume or mole or concentration etc.

So here measuring the mass of metal used in a reaction is a quantitative observation.