Correct Answer & Explanation
Option C
• The phrase 'By next month' indicates a completion of an action or state by a specific future time.
• The verb 'know' is a stative verb, meaning it generally does not take continuous forms.
• When a stative verb is used with a duration (for ten years) and a future completion point, the Future Perfect Tense (will have + V3) is used.
• This tense signifies that the state of 'knowing' will have been completed for a duration of ten years by next month.