Answer: b. Opening.
Explanation: In the given sentence we have the word "open" underlined, the best replacement for that word is "opening" the present participle of the verb, because this verbal form is used when we want to express two actions that are happening at the same time, like, in the given sentence, Trevor opened his chemistry book while he knew he was going to study the whole night. So the sentence would be: "Opening his chemistry book, Trevor knew he had a long night of studying ahead of him."
