Explanation:

- The sentence talks about what is needed to fight on the battlefield for one's country.

- "Strongness" is an incorrect form. The correct noun form is "strength," not "strongness."

- Option 1: "The greatest strongness" is incorrect because "strongness" is not a proper noun in English.

- Option 2: "A lots of strength" contains a grammatical error; it should be "a lot of strength." However, it still doesn’t fully capture the context of bravery needed for battle.

- Option 3: "No improvement" would mean keeping "strongness," which is incorrect.

- Option 4: "Great courage" is the most appropriate substitution. It emphasizes bravery needed in battle.

Option 4: Great courage is the correct choice.