Concept:This is a first conditional sentence.
It expresses a real possibility in the future.
Explanation:The pattern for a first conditional is: "If + present tense, will + base verb."
The given clause, "If your argument is not strong enough," uses the present tense.
Therefore, the main clause must use "will" with the base verb "forfeit."
So the completed sentence is: "If your argument is not strong enough, you will forfeit your claim to your father's property."
The other options do not fit this conditional structure.
Answer:C. will forfeit