The comparative sentence formation is required the following way of representation. For example, “hard, harder, and the hardest”.
In the above case, the best comparative word is “harder” to compare the two given sentences. The student thought the second test was harder (comparing) than the first test.
Thus, the options (c), (b) and (d) are incorrect.
In the above case, the best comparative word is “harder” to compare the two given sentences. The student thought the second test was harder (comparing) than the first test.
Therefore, the correct option is – a. harder.