0.
After "a", you need a noun here.
1.
Between "a", which is an article, and "19-year-old", which is a noun, the only possibility is to put an adjective. It is an adjective very similar to "young", but with a slightly different meaning.
2.
After "the most" we clearly need an adjective. This one means to be very well-known for doing bad things.
3.
After "the" and before "of", we clearly need a noun. Pay attention to the context, which says "Police are looking into...", which means investigating.