Which sentence is correctly punctuated?

Question $18$ options:

a$)$

Companies are looking for new forms of advertising, therefore many are using social networking sites such as Facebook and Twitter.

b$)$

Companies are looking for new forms of advertising; therefore, many are using social networking sites such as Facebook and Twitter.

c$)$

Companies are looking for new forms of advertising; therefore many are using social networking sites such as Facebook and Twitter.