Answer
The correct match of the variable $P\left( t \right)$ is option d. Population at time t.
Work Step by Step
Consider the model $P\left( t \right)={{P}_{0}}{{e}^{kt}},k>0$.
Here, ${{P}_{0}}$ is the population at time 0, $P\left( t \right)$ is the population at time $t$, $k$ is the exponential growth rate, and doubling time is the amount of time necessary for the population to double in size.
Thus, the variable $P\left( t \right)$ matches with option d.
