To determine which of the given numbers could be the probability of an event, we need to understand the range of values that a probability can take. The probability of an event is always a number between 0 and 1, inclusive. This means that the possible values for a probability $P$ are $0 \leq P \leq 1$.

Let's evaluate each number:

1. $1$ - This is within the range [0, 1].
2. $-0.48$ - This is less than 0, so it cannot be a probability.
3. $0.04$ - This is within the range [0, 1].
4. $1.56$ - This is greater than 1, so it cannot be a probability.
5. $0.21$ - This is within the range [0, 1].
6. $0$ - This is within the range [0, 1].

So, the numbers that could be a probability of an event are: $1, 0.04, 0.21, 0$.

Would you like more details or have any questions?

Here are five related questions to expand your understanding:

1. What is the definition of probability?
2. How do you calculate the probability of a single event?
3. What is the probability of an impossible event?
4. What is the probability of a certain event?
5. Can the sum of the probabilities of all possible outcomes of an experiment exceed 1?

Tip: Remember that probabilities are always between 0 and 1, inclusive. This range represents impossibility (0) to certainty (1).