Final answer:
Without a specific natural number, the remainder of dividing a number by 5 could range from 0 to 4, but since the number is divisible by 5, the remainder is 0.
Step-by-step explanation:
When Alex divided a natural number by 5, the remainder could be any number from 0 to 4, depending on what the natural number was. However, since the question does not give a specific number, we cannot determine the exact remainder. Therefore, the possible remainders when a natural number is divided by 5 are 0, 1, 2, 3, or 4. Each of these is a valid remainder because they are all less than 5, the divisor. Since the question indicates that the number is already divisible by 5, the remainder is 0 (option a).