Answer: Myself
Step-by-step explanation:
I have found the sentence that is missing in your question and I gave you the answer on it.
The sentence is: I was so proud of ______ when I ran a mile in under six minutes.
- The word that is missing and that can best fit in the blank is ''Myself'' because the subject is talking about own action. She\He is proud of himself\herself because she\he had run a mile.
- ''Myself'' is a pronoun if we are talking about parts of speech.