a) "Not really. I like to think that people make / take / get their decisions by themselves."
a) get (make a decision and take a decision are correct)
b) "Absolutely. The thing is: some strategies are really effective, but others are needy / poor / bad."
b) needy (you can say that some strategies are bad or poor)
c) "It depends. Some campaigns are successful in attracting / persuading / winning customers, but that's not necessarily true."
c) winning (you attract or persuade customers)