When my now 6 yo was 3, he was terrified of monsters. I would tell him, "I assure you there are no monsters in this house, but if it will make you feel better, I'll spray for them". I'd then make up some "anti-monster spray" (anything you can spray profusely & still like to smell works). I'd then spray down the areas of concern.
It's a tough call since all of the advice says "don't invalidate" and "don't say they exist". This was my attempt to let him know I took his feelings seriously without acting like monsters might be lurking in the closet.
For what it's worth, you daughter may be seeing or hearing something in the basement that = monster to her. We had to move some electronics that, to our son were, "red eyes in the backyard" (two red lights reflected in the window).
You may want to go down there with her (I'd hold hands and carry my spray) and ask her to tell you what she's seeing and hearing that makes her think there's a monster down there. And to be fair, I tend to be terrified of the basements with doorways...