The term `albedo` refers to the proportion of shortwave solar radiation that is reflected by a surface. Option 1, capacity to absorb solar radiation, is not correct because albedo specifically refers to the reflection of solar radiation, not absorption. Option 2, capacity to modify the path of solar radiation, is not correct either because albedo does not involve modifying the path of solar radiation, but rather reflects a certain amount of it. Option 4, amount of solar radiation returned by a surface, is also not correct as it does not accurately describe albedo.

Therefore, the correct answer is option 3, the proportion of the shortwave solar radiation reflected by a surface. Albedo is a measure of how much of the incoming solar radiation is reflected back into space. Surfaces with high albedo, such as snow or ice, reflect a large portion of the radiation, while surfaces with low albedo, such as asphalt or forests, absorb more of the radiation. Albedo plays a crucial role in determining the Earth`s energy balance and can have significant impacts on climate and temperature.