Answer
t=0,
The airline has most rapid increase of income in 2005
Work Step by Step
We compare points on the graph for the given t where the graph
is the steepest AND rising
At t=0, looking slightly ahead, the graph is rising
(although, strictly speaking, we should have data before t=0 to compare)...
At t=0.5 the graph is rising, less steep than for t=0
At t=1 the graph stops rising, not yet descending
At t=2 the graph descends
At t=2.5, still descending
At t=3.5, stops descending, hasn't begun to rise.
From the analysis, given the values for t,
we conclude that the graph is rising and steepest when t=0,
that is,
The airline has most rapid increase of income in 2005
