Which of the following is the correct security measure at the intersection of Confidentiality $-$ Transmission $-$ Policy & Procedures?

a$)$ Steps followed by small business owner when using mobile point$-$of$-$sales services

b$)$ Guidelines that outline data transmission mechanisms to ensure data is not modified during transmission

c$)$ Instructing the engineering staff to use hashing when storing files containing intellectual property

d$)$ A VPN protocol that provides secure communications