Explanation:

- Option 1: Alone, without assistance: This phrase refers to doing something independently, without any help from others. It has no relation to the concept of force or control that "an iron hand" implies.

- Option 2: Strict control: The idiom "an iron hand" means to govern or manage with firmness and authority, often suggesting harshness or severity. This is the correct interpretation of the idiom.

- Option 3: Memorising lessons: This involves committing information to memory, typically through repetition. It doesn't align with the notion of controlling or managing situations or people.

- Option 4: Straight talk: This means speaking honestly and directly, without evasiveness. It does not capture the aspect of command or control that is central to the idiom "an iron hand".