at the ease
A prepositional phrase begins with a preposition and ends with a noun. This is the only option that does this. At is a preposition. It tells where one thing is in relation to another. For example the dog is at the store. At tells where the dog is in relation to the store. Options A and D both begin with verbs. Smoothly is an adverb, but it does not contain a prepositional phrase. Option C is a dependent phrase that starts with a conjunction.