The image formed by a plane mirror is virtual, upright and the same size with the actual object. The upright image of an object in a plane mirror is can be found on the other side of the mirror which is why it is also virtual.
There are no options given in respect to the question and so it is not possible to choose. I would answer this question based on my knowledge and hope it satisfies you. he form of radio active decay that would be most likely detected by you if it were happening in the room next to the one you are currently standing in is gamma.
I will say increasing temperature but you dont have that option on your list, so I would take B. Increasing Concentration.