Answer: Development
Explanation: The parts of the sonata are exposition, development, and recapitulation.
In the development is where the "action" starts to occur, there are new themes and the melody can give some "improvisation" vibes. In this part, the themes presented in the first part, the exposition, are developed.
Usually, the themes that appear in the development are "broken" or turn "upside-down" which could make them hard to recognize, which is from where some of the tension comes.