The word "infested" most nearly means overrun.
To be infested is to be filled with or overrun by something, especially something harmful or unpleasant. In the case of the old mattress, it is overrun with bed bugs.
The other options do not have the same meaning as infested.
- "Complete" means to finish or accomplish something.
- "Diminish" means to make or become smaller or less in size, amount, or importance.
- "Guarantee" means to promise that something will happen or be the case.