The writer - Hellen Keller in the second paragraph gives the answer to the question in the penultimate and last sentence of the same. The referenced sentences indicate that it is impossible to experience happiness without understanding evil or contrasting its unfortunate events.
In order words, to be happy is a relative condition, one which is better appreciated by the realization that regardless of one's position, a more dire situation exists.
Hence the choice lies in the individual to choose to be happy regardless of their current situation. The answer, therefore, is D: Suffering leads to greater joy.
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