When two lenses are placed in contact, their powers can be added together to find the net power of the combination. The power of a lens is determined by its focal length, which is the distance at which it brings parallel rays of light to a focus. The formula to calculate power is P = 1/f, where P is power and f is focal length, measured in meters.

In this question, the first lens has a focal length of 50 cm, which is equal to 0.5 meters. Therefore, its power would be P1 = 1/0.5 = 2 dioptre.

Similarly, the second lens has a focal length of 25 cm, which is equal to 0.25 meters. Therefore, its power would be P2 = 1/0.25 = 4 dioptre.

Now, when these two lenses are placed in contact, their powers are added together to calculate the net power of the combination. P_net = P1 + P2 = 2 + 4 = 6 dioptre.

Hence, the correct answer is option 2, +6 dioptre.