Which sentence contains a dangling modifier? a) in a rush, the slow traffic was grating on my nerves. b) since i was in a rush, the slow traffic was grating on my nerves. c) i was in a rush, so the slow traffic was grating on my nerves. d) the slow traffic grated on my nerves because i was in a rus

The sentence that contains the dangling modifier is this: IN A RUSH, THE SLOW TRAFFIC WAS GRATING ON MY NERVES.
A dangling modifier refers to an ambiguous grammatical expression, that is constructed in such a way that, it makes it very easy for the grammatical modifier in the sentence to be associated with the wrong word or with no word at all. 
Sentences with dangling modifiers should be re-written in such a way that it will be very easy to recognize the word that  the modifier is modifying.
In the statement given in option A, it can be seen that it is very difficult to know who or what the rush is referring to; we can not say for sure may be it is the writer that is in a rush or may it is the slow traffic that is grating her nerves in a rushing manner.

The correct answer is option a.