first, we have to know what is electrolyte:
The electrolyte is a substance which breaks into cations + and anions - when dissolved in water.
now, to be able to classify electrolytes we have to look for:
1) If this chemical formula is one of the 7 strong acids (which are HCl, HBr, HI,
HNO3, HClO3, HClO4 and H2SO4) if the chemical formula one of them so,
this is a strong electrolyte.
2) when the chemical formula starts with H atom It's probably a weak
electrolyte. so here HBr is a strong electrolyte as it is one of the 7 strong
acids.
and HF is a weak electrolyte as it is a weak acid.
and Zn(NO3)2 is a strong electrolyte as it is soluble in water and form ions