Choose the correct one which is the weakest acid.

Higher the tendency to give ${\mathrm{H}}^{+}$ ion (i.e. to undergo ionisation), stronger will be the acid or vice-versa. 

HF has poor tendency to give ${\mathrm{H}}^{+}$ ion, hence it is the weakest acid among the given. The order of acidity of given acids is ${\mathrm{H}}_2{\mathrm{SO}}_4>{\mathrm{HNO}}_3>\mathrm{HCl}>\mathrm{HF}$.