Assertion : Conductivity of silicon increases by doping it with group-15 elements. Reason : Doping means introduction of small amount of impurities like P , As or Bi into the pure crystal.

Step-by-step solution

When a silicon crystal is doped with a group-15 elements, such as P, As, Sb or Bi the structure of the crystal lattice is left unchanged but an occasional atom with five valence electrons occupies a site that would normally be occupied by a silicon atom. The foreign atom uses four of its electrons in covalent bonding but the remaining fifth electron becomes delocalised and is thus free to contribute to electrical conduction.