The correct answer is (a).
There will be two images of an object placed between two parallel mirrors. The images will be upright and laterally inverted. The distance between the object and the first image will be equal to the distance between the object and the second image. The distance between the two images will be twice the distance between the object and either mirror.
The reason for this is that each mirror reflects the light from the object back to the other mirror, and the second mirror reflects the light back to the first mirror. This creates an infinite number of images, but only two of them are visible. The other images are behind the mirrors and cannot be seen.
Option (b) is incorrect because there will be two images, not one. Option (c) is incorrect because there will be two images, not six. Option (d) is incorrect because there will be two images, not infinite.