Kutuzov was very popular with the troops because he had proven 3 brave in battle.

1. The sentence describes a quality of Kutuzov. The options include 'HE'.
2. The structure 'he had proven X brave' requires an adjective or a noun phrase describing his bravery.
3. The word 'HE' is transformed into 'himself' to act as a reflexive pronoun, indicating that Kutuzov proved his bravery to himself or demonstrated it.
Final Answer:

himself