Answer:
Third Option - Most American car horns beep in the key of F. A listener with a good ear can tell a domestic-made car from a foreign-made car by listening to its horn.
Explanation:
The third option correctly separates the two different independent clauses in the sentence by using a period.
<u>First Clause:</u> Most American car horns beep in the key of F.
<u>Second Clause:</u> A listener with a good ear can tell a domestic-made car from a foreign-made car by listening to its horn.
Moreover, the verb indicating the subjetc's action must be in plural (<em>beep</em>, with no<em> s</em> at the end) given that the subject itself is plural (Most American car horns), and the adjectives describing the objects in the second clause - <em>domestic-made</em> and <em>foreign-made</em> - must have a dash prior to the describing verb (made) in order to make them compound adjectives.