+ Charge there means a H was taken off.
When a + Charge isn't there it's implied that the Carbon has 2 H's attached, and is bonded to 2 other carbons. With 4 bonds total it becomes sp3 -> which makes it not aromatic.
When a + Charge is there, all of a sudden now that carbon has only 3 bonds total (2 with neighboring Carbons, and 1 with the H that wasn't taken off). With 3 bonds total it becomes sp2 -> a requirement for being aromatic.
(C) satisfies all the criteria for being an aromatic
a) Cyclic Ring
b) All sp2, and NO sp3 carbons
c) 2, 6, 10, or 14 Pi electrons (this one has 6 since 3 double bonds).