Capacity refers to the amount of liquid or substance that a container can hold. In the given scenario, Nikki had a bucket full of 30 liters of water, which implies that the bucket has the capacity to hold 30 liters of water. Hence, the correct answer is option 'C' - Capacity.

Explanation:
The question is asking about what the statement tells us about the bucket. Let's analyze each option to determine the correct answer:

a) Weight: The weight of an object is the measure of the force of gravity acting on it. The statement does not provide any information about the weight of the bucket or the water in it. Therefore, option 'a' is incorrect.

b) Length: Length refers to the measurement of the longest dimension of an object. The statement does not mention anything about the length of the bucket or the water in it. Hence, option 'b' is also incorrect.

c) Capacity: Capacity refers to the maximum amount of liquid or substance that a container can hold. In this case, the statement explicitly states that the bucket is full of 30 liters of water. This information indicates that the bucket has a capacity of 30 liters. Therefore, option 'c' is the correct answer.

d) None of these: This option is chosen when none of the given options is correct. However, in this case, option 'c' (Capacity) is the correct answer. Therefore, option 'd' is incorrect.

In conclusion, the statement tells us about the capacity of the bucket, which is 30 liters. Hence, the correct answer is option 'c' - Capacity.