Answer: b. The most beautiful sunsets were the ones (that) light up the sky over the ocean.
Explanation: the subject of a sentence is the person or thing that performs the action of the verb. In the sentence "a" the relative pronoun "who" is referring to "the guide" who isn't the subject (the subject is "we"), in the sentence "c" the subject is "I" and "that" is referring to "The part of our vacation in Florida" which is the direct object. In sentence "d" the subject is "my family" and "that" is referring to "the vacation" which is the direct object. And in sentence "b", the subject is "The most beautiful sunsets" and "that" is acting as the subject of the verb "light up."
