The correct answer is D. I am reading about the Washington Monument because I hope to visit it.
Step-by-step explanation:
In grammar, the tenses refer to whether verbs are in the past, present or future. In the case of a sentence that includes two or more verbs, it is recommended these are in the same tense if they occur at the same time or show correctly when each action takes place. For example in the sentence "Yesterday, I went to the cinema, and today I am going to the beach" has two different tenses because one occurs in the past (yesterday), while other is occurring now or it is about to happen (today).
In the case presented, the sentence with the correct tense is "I am reading about the Washington Monument because I hope to visit it" because this sentence includes tenses in present "I am reading" and "I hope to visit" and this is correct because both actions can occur in the present, and by using present in both there is a consistency while other options mix tenses illogically.