Single Player[]
C4 is first seen in the level "Executive Order," where Mason uses it to blow up a wall to access the controls to the Soyuz 2. It is later seen in "The Defector," where Mason uses it to blow up the ceiling beneath a ZPU. It's final use in the level "Victor Charlie", where the player is tasked with covertly planting C4 in a Vietamese village and running to a safe spot. The player then detonates the C4 to ambush Viet Cong. The only time when the player can use it at will is in The Defector, when at the end of the mission where the player has to defend the docks. C4 can be found on a crate (like the claymores) and used normally to help defending.
Multiplayer[]
C4 is available in multiplayer for [1]2000. It can be bought after Create-a-Class is unlocked at level 4. The C4 in Black Ops is almost the exact same as the Modern Warfare and Modern Warfare 2 versions (featuring nearly the same skin, but lacking the flashing red light), but the player is armed with two C4s that can be placed and picked up after being deployed, unlike in Modern Warfare 2. They are deployed via the "aim" button, and can be detonated by either quickly double-tapping the reload button or using the detonator.