Given:
Depth in inches of an object as it sinks in the river is found by the equation

where t is measured in seconds.
To find:
The best description for the constant of proportionality.
Solution:
We know that, in the equation
, k is the constant of proportionality.
We have,

Here, constant of proportionality is 0.5.
Since, depth (y) in inches and time (t) in seconds, therefore the constant of proportionality id 0.5 inches per second.