Your answer is C. The word "respect" is abstract. You cannot count it, but you can only pay/earn it as you do with 'money', which is uncountable, too, but a concrete noun.
Nouns are the naming words of a person, place or thing. Abstract nouns are a type of noun that refers to a trait, quality, concept, idea, experience or any other entity. In other words, abstract nouns are the words which cannot be experienced through sensory organs.
Therefore, the sentence which contains an abstract noun is C. Respect must be earned. Wherein, <em>respect</em> is an abstract noun which cannot be experienced through five sense organs. In addition, it is an idea, concept or ideal to which it relates.
