First off, try to plug in the words and see if they make sense
A.) Benjamin Franklin was a brilliant inventor, also he never patented any of his designs.
B.) Benjamin Franklin was a brilliant inventor, yet he never patented any of his designs.
C.) Benjamin Franklin was a brilliant inventor, thus he never patented any of his designs.
D.) Benjamin Franklin was a brilliant inventor, namely he never patented any of his designs.
None of them but B makes sense:
Also is used when you're agreeing with the beginning fragment
Thus is used almost the same as "because" or "therefore"
Namely just doesn't make sense when you fit it in, shouldn't really be a hassle to discard this option out