The most effective way to correct the fragment in sentence 3 is:
C. Attach the fragment to sentence 2.
By doing so, we can establish a conection between <em>in 1959</em> and the following sentence and, for example, introduce a comma or a conjunction to join both sentences. One possibility could be:
<em>Improvements were made to the skateboard in 1959, when the first commercially made skateboard appeared.</em>
