Answer: B. It takes up the space like an "invisible" atom.
Explanation: Lone pair of electrons are the pair of non bonding electrons which does not take part in the bonding unless and until there is a need of it.
Lone pair of electrons do occupy some space and thus are not too small to affects the molecule's shape.
These never forces the molecules into a tetrahedral shape.
As shown in the water molecule, they are repelling the bonding pairs of electrons so as that the tetrahedral shape gets distorted and force the molecule to take the angular shape.