B) There is an unbelievable collection of diamonds on display at the Houston Museum of Natural Science.
This is the correct use of there because it is referring to a place, as in over there is the collection of diamonds. The collection is singular so the verb must be "is".
B) at the corner
Prepositions are words that tell you where one noun is in relation to another noun. "At" tells where the house is in relation to the corner. Prepositional phrases end with a noun or pronoun.
A) Sometimes body language may send negative messages.
In writing it is important to use active voice. The original sentence was in passive voice because the subject of the sentence (messages) was not actually doing the main verb (sent). In the correct answer, body language is the subject and it is doing the sending.
B) Sentences 2 and 3
Sentences 2 and 3 explain how body language can send messages which is what the passage is about.