To analyze the function on the interval from a to b, check this:
Where the function is respect to x axis.
What is the function behavior when the values of x gets bigger.
According to the graph, the function is located below the x axis on the interval from a to b, which means that the function is negative on this interval.
Also it is possible to observe that the greater the value of x, the smaller the value of f(x) (on the given interval). It means that the function presents an decreasing behavior on the interval from a to b.
Knowing this, it is possible to determine that the true about the function is that it is negative and decreasing.