In all the provided sentences, "fui" means "I was" or "I went," but the specific meaning in the context you're asking about can be clarified as follows:
- A. "Fui al médico" - "I went to the doctor."
- B. "Fui a Madrid" - "I went to Madrid."
- C. "Fui estudiante" - "I was a student."
- D. "Fui ayer" - "I went yesterday."
Among these, the sentence where "fui" clearly means "I was" is C. Fui estudiante.