Explanation:

- Hardly had I called him when my mother started screaming.

- The phrase "hardly...when" is a common structure in English, used to indicate that one action quickly follows another.

- The correct option here is when.

- Hardly had I called him so my mother started screaming.

- "So" is used to show cause and effect, but it doesn’t fit well after “hardly.”

- Hardly had I called him than my mother started screaming.

- "Than" is used for comparisons and doesn’t fit in this context.

- Hardly had I called him then my mother started screaming.

- "Then" indicates a sequence of events but isn’t used with "hardly."