21. You are preparing a test execution schedule for executing seven test cases TC 1 to TC 7.
The following figure includes the priorities of these test cases (1=highest priority, 3 = lowest priority).
The figure also shows the dependencies between test cases using arrows. For instance, the arrow from TC 4 to TC 5 means that TC 5 can only be executed if TC 4 was previously executed.

Which test case should be executed sixth?
Explanation / Rationale
We want to run test cases according to their priorities, but we also need to consider the dependencies.
If we only consider priorities, we want to first run TC 5 and TC 7 (highest priority), then TC 1, TC 3, and TC 4, and finally TC 2 and TC 6 (lowest priority).
However, in order to run TC 7, we need to first run TC 4.
In order to run TC 5, we need to run TC 4 and TC 2, but TC 2 is blocked by TC 1, which should be run prior to TC 2.
So, in order to run priority 1 test cases as early as possible, the first five test cases should be: TC 4 - TC 7 - TC 1 - TC 2 - TC 5.
Next, we need to run TC 3, because it has higher priority than TC 6.
Thus the full schedule will be TC 4 – TC 7 – TC 1 – TC 2 – TC 5 – TC 3 – TC 6.
So, the sixth test case will be TC 3.
Thus:
a) Is correct
b) Is not correct
c) Is not correct
d) Is not correct