large-sized chlorine atoms do not fit in between the small boron atoms whereas small-sized hydrogen atoms fit between boron atoms
Hydrogen is of small size, so it easily forms a bridge without caustic steric hindrance whereas the chlorine size is larger than boron. Hence, it is not able to form a bridge. So, BCl3 does not exist as a dimer due to large size of chlorine.
Therefore, BCl3 does not exist as dimer but BH3 exists as a dimer (B2H6) because of large size of chlorine
So, the correct option is (C)