The target heart rate is strictly increasing then strictly decreasing between 40 and 60 minutes (Option B)
There are three observed intervals shown on the graph and during those three there are times when his heart rate increases, then becomes stable and then decreases.
However, between the intervals of 40 to 60 minutes, there was no point where it was stable or level. It increased rapidly at the 40th minute and at the 50th minute it began to drop rapidly till the 60th minute.
**(Note that when he approached the 70th minute he was already stable and there was no increase or decrease)**