I have a 1095-B and a 1095-C for the exact same health insurance policy, but the 1095-C has me as covered for one less month than the 1095-B. Which form should I use?

I originally received a 1095-B from the health insurance provider I had at the end of 2015; I had the health insurance plan with them through my employer. That 1095-B says I had health insurance for the last 3 months of 2015.

I later received a 1095-C from my employer, regarding that exact same health insurance policy, but that form seems to state that I only had health insurance for October and November of 2015. The code for Box 14 on this 1095-C for the month of December is "1H",  meaning "No offer of coverage", and Box 15 has no contribution amount listed for December.

I am confused. Which form should I use as proof of insurance to the IRS? I'm assuming copies of both the 1095-B and 1095-C were sent to them.