The correct answer is D. absolute phrase.
Let's use the system of elimination here:
A is incorrect because there are no infinitives in this phrase. An infinitive looks like this: to look, to sing, to dance, to sleep, etc.
B is incorrect because the prepositional phrase is only up the river - but not the entire phrase.
C is incorrect because the phrase doesn't have a finite verb - in order for it to be a sentence fragment (aka a dependent clause), it has to contain a finite verb.
Therefore, the correct answer is D. An absolute phrase is a phrase that doesn't contain a finite verb, but does contain a participle (flowing up).