Select the most appropriate option to fill in the blank.

Hardly had I called him ______ my mother started screaming.

when

The correct option to fill in the blank is when.

The sentence is using the hardly...when construction, which is used to express two actions that happened very close together in time. In this case, the speaker called the person and then their mother started screaming. The hardly...when construction emphasizes the short amount of time between the two actions.

The other options are not as appropriate.

• Then means at that time. This is not the right word to use, as it does not convey the same sense of urgency as "when."
• So means therefore. This is not the right word to use, as it does not make sense to say that the speaker called the person so their mother started screaming.
• Than means compared to. This is not the right word to use, as it does not make sense to say that the speaker called the person than their mother started screaming.