Two concave lenses L1 and L2 are kept in contact with each other. If the space between the two lenses is filled with a material of smaller retractive index, the magnitude of the focal length of the combination

The focal length of the combination of lenses is determined by how much a ray of light bends while passing through one medium. The more it bends, the shorter the focal length.
When the space between the two concave lenses is filed with a material with a refactive which is grater than that of air but less than that of the glass used the ray will bend in the same direction wrt the normal at each interface, but not as much as before, because the difference in refractive indices is less. So, the lens combination will be weaker, with a longer focal length