The correct option is C He is sure to fall.
A clause is a part of a larger sentence containing a subject and a predicate of its own.
A Principal Clause is an independent clause which can exist even without a subordinate clause. A Subordinate Clause is called a dependent clause because it cannot exist without the help of the principal clause. It begins with a subordinating conjunction.
Option C is correct as 'He is sure to fall' is the principal clause. It can exist on its own. It contains its own subject and predicate.
Option A is incorrect as it is a phrase and not a clause.
Option B is incorrect as it is the subordinate clause as it begins with the subordinating conjunction 'that'.
Option D is incorrect as it is a part of the principal clause, not in entirety.