Answer:
B. The sick need to be treated.
Step-by-step explanation:
Although it's more of a statement than a definition, "The sick need to be treated" is the closest thing to a definition in your choices.
A definition is something every agrees on to define something, it's based on facts, not on opinions, judgement nor opinions, which can vary from one individual to another. (like: a table is a flat surface raised in the air through the support of legs)
All other choices (A, C and D) have a judgement call or an opinion in them (believe, deserve). Those might apply to you, but not to your neighbor, so they cannot be considered as a definition.