The view state in Visualforce represents the state of the page - including the components, field values, and controller state. This error usually occurs when the page's view state becomes too large. To resolve this issue:
C: Ensure variables are marked as transient: The transient keyword in Apex can be used to declare instance variables that should not be part of the view state and therefore not be saved. This is especially useful for large data that does not need to be maintained across postbacks.
Marking properties as private (Option B) or ensuring queries do not exceed governor limits (Option A) might be best practices, but they do not directly impact the view state size. Ensuring profiles have access to the Visualforce page (Option D) is a matter of visibility and access, not related to view state errors.
References:
Salesforce Developer Documentation on View State: Visualforce View State