Explanation:

- Option 1: at all

- This part is grammatically correct. "At all" is often used to emphasize a point. Here it emphasizes the old man's ease in climbing.

- Option 2: climbed

- "Climbed" is the past tense of "climb" and fits correctly in this context, indicating a completed action.

- Option 3: hardly no effort

- This contains a double negative, "hardly" and "no," which makes it incorrect. The correct form should be "hardly any effort" to maintain the intended meaning.

- Option 4: The old man

- This phrase is a correct noun phrase, identifying the subject of the sentence.

- Correct Answer: Option 3 - hardly no effort