Answer
The only point, out of the given five points, where the function decreases is at $t=2.5$.
The airline's most rapid decrease of revenue was around the summer of 2007.
Work Step by Step
At $t=0$ $P(t)$ is increasing.
At $t=0.5$ $P(t)$ is also increasing.
At $t=1$ $P(t)$ is changing from increasing to decreasing, but it doesn't have a steep decrease after that.
At $t=2.5$ $P(t)$ is decreasing.
At $t=3.5$ $P(t)$ stops decreasing, but is not yet increasing.
As the only point, where the function decreases is at $t=2.5$, it is the answer.
2.5 years after 2005 means, that it is around the end of summer of 2007.
