Whilst any explosive in public can be a potentially unsafe compound, C4 is probably one of the better choices. + It is really difficult to detonate accidentall

Whilst any explosive in public can be a potentially unsafe compound, C4 is probably one of the better choices. + It is really difficult to detonate accidentally. (Even when it burns it may not detonate.) + It can be shaped, so that the charge only moves in the intended direction. (So the first floor isn't at immediate risk.)