(The answer is at the bottom)
A) Medical studies have proven that sun damage can lead to melanoma; however, you should always wear sunscreen to protect your skin from those damaging ultra-voilet rays.
It's pretty much starting another sentence however isn't the best choice.
B) Medical studies have proven that sun damage can lead to melanoma; finally, you should always wear sunscreen to protect your skin from those damaging ultra-violet rays.
"Finally" is the words of chronological order, and this is sentence is not in chronological order.
<u>C) Medical studies have proven that sun damage can lead to melanoma; </u><u>therefore, </u><u>you should always wear sunscreen to protect your skin from those damaging ultra-violet rays. </u>
<u>this makes the most sense and fills in the blank correctly. It fits with the way the sentence is arranged.</u>
D) Medical studies have proven that sun damage can lead to melanoma; for example, you should always wear sunscreen to protect your skin from those damaging ultra-violet rays.
this doesn't even fit the sentence. It makes NO sense.
C is the BEST answer.