He _____ to contact the nearest police station for the last twenty minutes?

Was trying

Has been trying

Kept trying

None of these

"He has been trying to contact the nearest police station for the last twenty minutes."

The time frame "for the last twenty minutes" indicates a continuous action that began in the past and continues up to the present.

This makes the present perfect continuous tense (has/have been + verb - ing) the most suitable choice.