Correct Answer: within

Solution : The first option is the correct choice.

The meanings of the given options are:

• Within : inside the range of a specified action or perception.
• Before : during the period of time preceding a particular event or time.
• Between : at, into, or across the space separating two objects or regions.
• Along : moving in a constant direction on a road, path, or any more or less horizontal surface.

As per the context of the sentence, the artists are going to arrive in or earlier than an hour .

Therefore, the complete sentence is: "The artists will arrive within an hour."