The correct answer is A. Weather.
The relationship between “mother” and “child” is that a mother gives birth to a child. The relationship between “cloud” and “weather” is that clouds are a major factor in the weather.
Option B, “rain,” is not a correct answer because rain is a product of weather, not a factor in it. Option C, “thunder,” is not a correct answer because thunder is a sound that is sometimes associated with lightning, which is a product of weather. Option D, “water,” is not a correct answer because water is a substance that can be found in clouds, but it is not a factor in the weather.