${\mathrm{BCl}}_3$ does not exist as dimer but ${\mathrm{BH}}_3$ exist as dimer because

Cl is more electropossitive than H

There is $\mathrm{P\pi } – \mathrm{P\pi }$ back bonding in ${\mathrm{BCl}}_3$ but ${\mathrm{BH}}_3$ does not contain such multiple bonding

Large chlorine atoms do not fit between small sized boron atoms whereas small hydrogen atoms get fitted between boron atoms

None of these