Final answer:
The Great Library of Alexandria was most likely destroyed by a combination of events, the most significant of which was a fire during Julius Caesar's siege. However, its total destruction was a gradual process with no single event or deliberate act in the fifth century A.D. definitively confirmed as the cause.
Step-by-step explanation:
The Great Library of Alexandria was destroyed in a series of unfortunate events over several centuries. The most significant of those was likely the incident in which parts of the library were accidentally destroyed by fire during the siege by Julius Caesar in 48 BCE. However, the complete destruction of the library is thought to be a gradual process, with subsequent events, such as the edict of Theophilus in 391 CE against pagan temples, and later the Muslim conquest, contributing to its demise. None of these events directly relates to being abandoned after Alexander the Great's death, destroyed by a Nile flood, or being deliberately destroyed by people in the fifth century A.D. as the question suggests.
Despite various accounts and theories surrounding its destruction, the exact cause remains a topic of historical debate, further complicated by a scarcity of contemporary accounts. There are also competing versions attributing its decline to either Christian or Muslim actions centuries later. Therefore, the exact cause of the Library's destruction cannot be definitively stated from the provided options, which seem to be a mixture of speculations or related to other historical contexts.