Light travels in a straight line, known as rectilinear propagation of light, when it passes through a medium that is homogeneous and transparent. Let`s dissect each option to understand why the correct answer is option 3.

Option 1: If the medium has a variable refractive index, the path of light will bend due to the change in the refractive index. Therefore, light will not travel in a straight line.

Option 2: If the medium is made up of slabs of different refractive indices, the light will refract or bend each time it enters a new slab. As a result, the path of light will not be straight.

Option 3: This is the correct answer. If the medium is both homogeneous and transparent, the light will travel without any interruption or change in direction. Homogeneity ensures that the medium has the same properties throughout, and transparency allows light to pass through without being absorbed or scattered.

Option 4: In an inhomogeneous medium, the refractive index varies at different points. This variation in the refractive index will cause the light to bend, resulting in a non-straight path.

In conclusion, the statement that light travels in a straight line holds true when the medium of travel is homogeneous and transparent, making option