The correct option is C Life is an unending struggle
The idea that life is an unending struggle (option C) is affirmed by the sentence "Life is one continuous, competitive examination". An unending struggle is synonymous to a continuous examination. The struggle refers to the constant efforts one makes to win in something, here, in life. Hence, option C is correct.
The passage does not focus on a life that is full of pain and suffering (this gives the passage a negative undertone), it speaks about the necessity of pain (in order to feel the joy when that pain is gone). Hence, A is incorrect.
The idea in B (struggle or no struggle) does not represent the passage. In fact, the passage concentrates on the importance of struggle; with struggle there is joy: "Life indeed would be dull if there were no difficulties." Option B contradicts this line, hence, it is incorrect.
Similarly, option D completely contradicts the passage. Lines like "there are no gains without pains.", "No victory is a real triumph unless the foe is worthy of the steel." (meaning that if there are no struggles, victory has no value) prove that option D does not represent the idea of the passage. Hence, D is incorrect.