The C Story is your Monster Of The Week or Case Of The Week. The C Story is 'what this week's episode is about'. Remember, your C Stories will be resolved in each book.
The TV Guide is a great place to see descriptions of C Stories. The description of the C Story is how the Guide draws viewers to shows. http://www.tvguide.com/Listings/
Here are a few exmples from today's listings:
When one half of a serial killer duo commits suicide, the team must identify and find the other killer, who had abducted his next victim.
Andy and Nick cross paths with a powerful drug dealer while investigating a death threat aimed at a priest. Meanwhile, Chris and Swarek argue over what it takes to be a good cop.
Jess babysits Russell's daughter for the weekend and has her first enounter with his ex-wife. Elsewhere, a pregnancy scare sends Cece and Schmidt into a panic, and Nick dates a younger woman.
Here are a few examples of episode descriptions from the first season of Sons of Anarchy
A new deputy police chief arrives in Charming, and Jax suggests a non-violent way to throw him off Samcro's tail. Meanwhile, Opie is forced to make a difficult decision over his relationship with the Sons when Donna faces financial hardship.
Samcro members take arms to capture an assailant after the daughter of a prominent Charming family is assaulted. Meanwhile, Tara's ex-boyfriend pays a visit.
The club head to Nevada to help out a brother club, the Devil's Tribe, after they receive threats from the Mayans. Meanwhile, Tara's ex-boyfriend begins investigating Samcro.
So, you can see that the C Stories are the events that concern your characters in each story.
With A, B, and C stories, you have the making of a series.