Which of the following pairs shows highest bond dissociation enthalpy among halogens and lowest bond dissociation enthalpy among hydrogen halides? 

NO ANSWER 

   In Hydrogen halides
Down the group affinity of hydrogen towards the halogen atom decreases as the size of the halogen atom increases
$\therefore$ Thermal stability & BDE of  $\mathrm{HF}>\mathrm{HCl}>\mathrm{HBr}>\mathrm{HI}$
In halogens
Down the group as the size of halogen increases X – X bond enthalpy must decrease.
$\therefore$     Expected trend
 ${\mathrm{F}}_2>{\mathrm{Cl}}_2>{\mathrm{Br}}_2>{\mathrm{I}}_2$
But due to very small size of fluorine atoms,  $:\underset{··}{\overset{··}{\mathrm{F}}}-\underset{··}{\overset{··}{\mathrm{F}}}:$   the repulsions between the lone pairs of adjacent atoms is very high.  
 $\therefore$   F – F bond enthalpy is abnormally low. 
Experimental Trend ${\mathrm{Cl}}_2>{\mathrm{Br}}_2>{\mathrm{F}}_2>{\mathrm{I}}_2$