C is the only right answer. There's no such thing as a "strait pin." It is a pin that is straight, so it is a straight pin.

The other three are correct because they all make reference to "straits"--very narrow bodies of water. A is a specific one, so that's a no-brainer. B refers to being in a very tight situation--a dire strait. D refers to being in a very tight garment--a straitjacket. People mix these last two up with "straight" because a dire situation or a tight jacket would squeeze you to straightness, but that is not what the words were referring to when they were coined.