For electronics, rosin core is best. Definitely don't use acid core, which is meant for plumbing. The most common type of rosin core solder for electronics usually consists of a mix of 60% tin & 40% lead, often referred to as 60/40 Rosin Core solder.
You may see slight variations regarding the mix ratio of tin/lead as well, with a 63/37 mix ratio instead of 60/40. Usually it is more common to find a 60/40 rosin core, but either is fine with a slight preference to the 63/37. If you look at a chart for the melting and cooling properties of solder, you will see that the 63/37 does not go into what is called a "plastic state" as it cools. What that is, as the solder cools after the heat from the iron is removed, the solder will have a frosted look and will not make a very good joint. It will not be as shinny as it should look.
Never use the acid core for electronic work. The acid left behind will absorb water from the air and eat away at the circuits. The acid flux is mainly used for copper water pipes.