Final answer:
The word 'alleged' means that someone is saying that a visitor to the national art gallery is responsible for damaging the 18th-century painting, but it has not been confirmed.
Step-by-step explanation:
A visitor to the national art gallery is alleged to have damaged an 18th-century painting.
The word 'alleged' means that it is claimed or accused, but it has not been proven. In this context, it means that someone is saying that a visitor to the national art gallery is responsible for damaging the painting, but it has not been confirmed. Similarly, past thefts from museums, some carried out by professionals with insider assistance, and others by more opportunistic individuals, have highlighted the vulnerability of such cultural treasures.
For example, if someone says 'John is alleged to have stolen the money,' it means that people are saying that John stole the money, but it has not been proven.