Soporific adj. (ˌsä-pə-ˈri-fik) causing a person to become sleepy which sentence uses the definition of the word soporific given in the dictionary entry?
the main event was soporific; the arena was abuzz with energy and excitement.
the medication i had to take was soporific, so i had to be careful not to drive my car while taking it.
after each of my soporific dreams, i wrote in my journal what i remembered in order to see if there were any patterns.
as a result of its soporific plot, i couldn’t take my eyes off the screen until the very end of the movie.

The second sentence uses the definition of the word.

Explanation :

In the second sentence the subject takes a medication that makes him/her "soporific", meaning it promprs him/her to sleep. Hence, the person has to avoid driving since it can be dangerous.  In the rest of the example there is no logical connection between the noun affected by "soporific" and the predicate: in all the other cases the effects stated are those of excitement, accelaration, and enhancement.

the answer is b. the tomb is underground and dark, and its opening is covered by a heavy grate.

i would say a because the main idea is what the author is the point the author is trying to get across.