First of all, you need to understand what antecedents and pronouns are, and what the relationship between the two is. A pronoun is a word used to replace the noun in front of it. An antecedent is that particular noun in front of the pronoun, to which the pronoun refers. Thus, here are my answers:
10. William asked for his coffee to be brought to the conference room.
The pronoun here is the word HIS, and given that it refers to William (it is William's coffee after all), it means that the antecedent here is WILLIAM
11. The music was so loud that it hurt our ears.
The pronoun here is the word OUR, however, there is no antecedent in this sentence; it is implied that the antecedent is WE (however it is not stated directly in the sentence itself)
12. Thomas and Christine forgot to bring their umbrellas to work.
The pronoun here is the word THEIR, and given that it refers to Thomas and Christine (those are their umbrellas after all), it means that the antecedents here are THOMAS AND CHRISTINE
13. Since Grandma Helen was ten years old, she had been writing letters.
The pronoun here is the word SHE, and given that this particular pronoun was used to replace the noun Grandma Helen, it means that the antecedent is GRANDMA HELEN
14. The rain pounded on the roof and it leaked through the cracks.
Here, the pronoun is the word IT, and given that it refers to the rain (it was used to replace the word rain in the latter part of the sentence), it means that the antecedent is RAIN
15. His messenger bag was so full that Randy could not fit another file in it.
Although there are two pronouns here (his and it), we cannot choose his as the correct answer as it does not have an antecedent (remember that antecedents are in front of the pronoun, and here Randy is located after it). This means that the pronoun we will use is IT, and its antecedent is HIS MESSENGER BAG
16. Mr. Cooper told his students that they were responsible for their grades.There are couple of answers here, given that there are a couple of pronouns to choose from. The first pronoun and its antecedent are HIS (pronoun) and MR. COOPER (its antecedent). Then we have the pronouns THEY and THEIR along with their antecedent HIS STUDENTS.