Remain silent again at the second prompt; do not reveal your presence.
Step-by-step
At the first visitor choice, select 'Remain silent'.
At the second response, select '...'.
Because you never made a sound, Mrs leaves and Ending 1 triggers.
If you get a different ending
If you spoke, locked the door, or asked another question and then chose silence, you already revealed your presence and the same silence choice leads to a Game Over instead.