When two convex lenses are kept in contact with each other, the combined lens system is basically a single lens with an equivalent power. To find the focal length of this combined lens system, we can use the lens formula:

1/f = (1/f1) + (1/f2),

where f is the focal length of the combined lens system, f1 is the focal length of the first lens, and f2 is the focal length of the second lens.

In this case, both lenses have a power of 2 dioptre. The focal length (f1 and f2) of a lens can be calculated using the formula:

f = 1/p,

where p is the power of the lens in dioptre.

So, the focal length of each lens is 1/2 = 0.5 m.

Substituting these values into the lens formula, we get:

1/f = (1/0.5) + (1/0.5) = 2 + 2 = 4.

Taking the reciprocal of both sides of the equation, we find that the focal length of the combined lens system is 1/4 = 0.25 m.

Therefore, the correct answer is option 4: 0.25