The Primer

Primer:

1. A textbook formerly used in primary education to teach the alphabet and other basic concepts.
2. (biology) A single-stranded nucleic acid molecule required for the initiation of replication of a DNA molecule.
3. Any substance used to start a fire.
4. A small charge that explodes when struck with force that ignites the gunpowder in ammunition.
5. A layer of paint designed to underlay a topcoat, used to enhance the adhesion and durability of the topcoat and help in protection of the surface.
6. A device used to circulate gasoline into the ignition chamber of an engine.

Which one?