Google looks down upon paid links for enhancing page rank. If a website sells links, what action/s does Google recommend to avoid being penalized?

a. The text of the paid links should state the words “paid text link” for Google to identify it as a paid link
b. Only Paid text links to non profit websites should be accepted
c. Paid links should be disclosed through the “rel=nofollow” attribute in the hyperlink
d. Paid links should be disclosed through the “index=nofollow” attribute in the hyperlink