In ITIL 4, an incident is defined as an unplanned interruption to a service, or a reduction in the quality of a service. Unplanned maintenance that results in a service being unavailable for two hours clearly represents an unplanned interruption, so it is managed as an incident.
The incident management practice is responsible for:
Managing the lifecycle of incidents
Restoring normal service operation as quickly as possible
Minimizing the negative impact on business operations
That makes incident management the practice most likely to be involved in handling and coordinating communication about this unplanned downtime.
Why the others are less appropriate here:
Problem management aims to manage causes of incidents and reduce their likelihood or impact over time; it does not primarily manage the immediate unplanned outage.
Change enablement manages changes (including emergency changes), but the question is focused on managing the service unavailability to users, which falls under incident management.
Service request management deals with pre-defined, user-initiated requests (such as password resets, information requests), not with unplanned outages.
Therefore, the practice most likely to manage this unplanned unavailability is incident management, so A is correct.