What are call provisions and sinking fund provisions? Do these provisions make bonds more or less risky?

Question:

MINI CASE

Sam Struthers and Shawna Tibbs are vice-presidents of Mutual of Seattle Insurance Company and co-directors of the company's pension fund management division. A major new client, the Northwestern Municipal Alliance, has requested that Mutual of Seattle present an investment seminar to the mayors of the represented cities, and Struthers and Tibbs, who will make the actual presentation, have asked you to help them by answering the following questions. Because the Boeing Company operates in one of the league's cities, you are to work Boeing into the presentation.

Bonds
When companies need to raise money, issuing bonds is one way to do it. A bond functions as a loan between an investor and a corporation. The investor agrees to give the corporation a specific amount of money for a specific period of time in exchange...