input: Write a python function to find the key with the highest value in a dictionary.
  assert most_frequent({"a": 10, "b": 20, "c": 5}) == "b"
  assert most_frequent({"apple": 1, "banana": 3, "cherry": 2}) == "banana"
  assert most_frequent({"x": 100, "y": 200, "z": 300}) == "z"
output: def most_frequent(d):
      return max(d, key=d.get)