The capacity of a bucket can be measured in kilograms (kg).

Explanation:
To understand why the correct answer is option 'D' (kg), let's first understand what capacity means in the context of a bucket.

Definition of Capacity:
Capacity refers to the maximum amount of liquid or substance that a container can hold. In the case of a bucket, capacity represents the volume of liquid or material it can contain.

Units of Measurement:
Different units can be used to measure capacity, such as liters (L), milliliters (mL), gallons (gal), pounds (lb), or kilograms (kg). However, when measuring the capacity of a bucket, liters (L) or kilograms (kg) are the most commonly used units.

Why Kilograms (kg) is the Correct Answer:
1. In the context of a bucket, kilograms (kg) is used to measure the capacity when the bucket is intended to hold a substance with a specific weight. This is often the case with solid materials, such as sand, gravel, or rice.

2. The weight of a substance can directly determine the capacity of a bucket. For example, if a bucket is designed to hold 5 kg of sand, its capacity is 5 kg.

3. Kilograms (kg) is a unit of weight or mass, and it is commonly used to measure the weight or mass of objects or substances. It is a more appropriate unit for measuring the capacity of a bucket when dealing with materials that have a specific weight.

Alternative Units:
Although kilograms (kg) is the most suitable unit for measuring the capacity of a bucket in this context, it is worth mentioning that other units, such as liters (L), can also be used for measuring the capacity of a bucket when dealing with liquid substances.

However, as the question does not specify the type of substance the bucket is intended to hold, kilograms (kg) is the safest and most appropriate answer in this case.

In conclusion, the capacity of a bucket can be measured in kilograms (kg) when dealing with materials that have a specific weight.