The answer is (c); the image in a plane mirror is inverted front-to-back, and in no other direction. Because the transparency under consideration here has no depth, this bit of information is irrelevant, as can be seen in the photograph below.
Notice that the word is not inverted either in the top-to-bottom or the left-to-right directions. One reason for possible confusion here is that most people are used to holding up text with an opaque background in front of a mirror, in which case the image reads backwards. However, if you examine the situation carefully you will notice that in order to see the writing in the mirror the page had to be reversed, so even in this case there is no left-to-right inversion created by the mirror.
Notice, in the photograph above, that Gwen's hand is indeed reversed front-to-back!!
