Question

the root meter means measure. what does the word hydrometer mean?
a tool scientists use to listen to sounds made underwater
a tool scientists use to measure how dense a liquid is
a tool scientists use to study living organisms in the water

Explanation:

To determine the meaning of "hydrometer", we analyze the word's roots. "Hydro-" means water (or liquid) and "meter" means measure. So we look for an option related to measuring liquid. The first option is about listening to underwater sounds (not measuring), the third is about studying water organisms (not measuring). The second option is about measuring liquid density, which fits "hydro-" (liquid) + "meter" (measure).

Answer:

a tool scientists use to measure how dense a liquid is