Which sentence uses punctuation correctly?

a.
I just read an excellent article about climate change; however the author definitely had a bias on one side of the debate.

b.
I just read an excellent article about climate change; however, the author definitely had a bias on one side of the debate.

c.
I just read an excellent article about climate change however; the author definitely had a bias on one side of the debate.

d.
I just read an excellent article about climate change, however; the author definitely had a bias on one side of the debate.