Answer:
The answer is: After noticing his cleat was untied, the runner stopped to tie his laces.
Step-by-step explanation:
A complex sentence is a sentence that consists of an independent clause and a dependent clause.
An independent clause is a clause that can stand on its own, while the dependent clause is a clause that cannot stand on its own. In a complex sentence, these clauses are connected to each other by "subordinating conjunctions."
Subordinating conjunctions include (but not limited to) the following: after, although, because, before, that though, till unless, while, when whenever, etc.
In the sentence, "After noticing his cleat was untied, the runner stopped to tie his laces," the independent clause is "the runner stopped to tie his laces" while the dependent clause is "After noticing his cleat was untied."
Since the sentence consists of both independent and dependent clauses and these are connected by a subordinating conjunction, "After," then it is the only complex sentence among the choices.