The correct answer is “to force the audience to use its imagination”.
Wilder wanted to challenge the audience to the see the play for what it was – a play. He wasn’t a fan of the common convention of real-life theatre. It was popular at the time to fill the stage with artifacts to make it seem like there was a specific time and place in the play, carrying the audience on a journey. Instead, Wilder rejected that aspect, he did not wish for the illusion of real life, as he actively encouraged the audience to see the artifice.