Answer:
C
Explanation:

Take the reciprocal of both sides:

You can now imagine a right triangle where the length of the side opposite this angle has a length of 4 and the hypotenuse has a length of 5. The third side by the Pythagorean theorem must therefore have a length of 3. The cosine of an angle is the length of the adjacent side divided by the length of the hypotenuse, or in this case:

Therefore, the correct answer is choice C. Hope this helps!