According to the gestaltists' idea of transposition, if an animal is trained to approach a medium gray card and to avoid a black card, and then is presented with a medium gray card along with a white one, the animal will tend to:

approach the white card

Explanation:

According to gestalitist idea of transposition, organisms tend to discriminate the stimulus that resulted in negative situation in the past and will transfer that discrimination to other similar situation in the future.

In the excerpt above, we know that the animals have been trained to approach medium card and avoid black card. When we presented white card to them, they do not have enough information to know whether the new card will resulted in negative situation or not. Because of this, they tend to approach the new card in order to collect those information.