Answer and explanation:
First, it is necessary to state what is a relative clause and how to use each one in this assessment to better understand them.
Relative clauses are non-essential part of a sentence: if they are removed from the sentence it will not change significantly the meaning.
There are defining relative clauses that we use to emphasize the person or thing we are talking about in a sentence. If we remove them, the sentence would change its meaning greatly, and they are not followed by commas nor any other punctuation marks.
There are also non-defining relative clauses, which we use to add additional information about the person or thing we are talking about without changing the overall meaning of the sentence if we remove them. They are separated from the rest of the sentence with commas or parentheses.
The most common relative clauses are who (people), where (place), which (animals, objects, etc.) and that (can replace who or which). There are some others like whom and whose.
Now we check and examine the answers:
'The day, which started out as disappointing, eventually turned out to be great. First, I went to work, where I learned that I was scheduled to work over the coming weekend, even though I had asked for time off. I cleared the matter up by speaking to my boss who was very understanding. Cindy, to whom I will now owe a huge favor, agreed to work my shifts. Then, I went to lunch with friends at the diner that serves my favorite milkshakes. It turned out to be wonderful day!'
- In the first part we use 'where' because additional information about a place (work) is indicated.
- Then we use 'who' because relevant information about a person is indicated and cannot be separated from the main sentence (in this case, the boss).
- After that, there is 'whom', that it is used to indicate extra information about the object of the sentence (the one who receives the action from the verb, in this case, Cindy).
- Finally, we use 'that' instead of 'where' because even when important information about a place (the diner) is indicated, we cannot use 'where' without a subject.
