Final answer:
A population with a per capita growth rate of 0.15 is most likely to increase in size in the short term, as this indicates a higher birth rate compared to the death rate.
Step-by-step explanation:
If a population has a per capita growth rate, r, of 0.15, the most likely statement to be TRUE is that its size will increase in the short term. This positive growth rate indicates that the population is growing, which means that the number of births and individuals immigrating into the population must exceed the number of deaths and individuals emigrating out of the population.
The birth rate (b) and the death rate (d) are the two main factors affecting population growth, and if the population growth rate (r) is positive, it generally implies that the birth rate is higher than the death rate. Thus, we can understand that option a, the population size increasing, aligns with the reality of the positive growth rate.