Words that are similar to each other in meaning are also known as synonyms of each other. The word ‘Alleviate’ means to reduce a problem or issue, or to lessen it significantly. While 'Eradicate' and ‘Remove’ are tangentially related to the word, and ‘Understand’ is irrelevant to the word in terms of meaning, 'Lessen' is the closest in meaning to ‘Alleviate’ – leaving us with the correct answer, Option D).