The observation, in this case, is quantitative.
Quantitative observations are observations that can be recorded based on quantitative data. In other words, they are observations that can be assigned numerical values.
Quantitative observations are as opposed to qualitative observations because the former cannot be assigned numerical values. They can be ranked or qualified.
In this case, Danielle and Heather could assign numbers to the length, width, and height of the tank in order to calculate its volume.
More on quantitative observations can be found here: https://brainly.com/question/17491501