Answer:
a. P atoms introduce additional mobile negative charges
Explanation:
Silicon atoms have a valency of 4, this means that there are 4 electrons in their outermost orbital. Thus, in silicon crystals, each Si atom is connected to 4 different Si atoms.
When we replace a few of these Si atoms with P atoms, which have a valency of 5, 4 out of these 5 outermost electrons will be bonded with the surrounding Si atoms. The fifth electron would not be bonded, meaning it would be free to move, acting as a mobile negative charge carrier (See the picture below).
Just to be clear, regarding option c), it's true that Ge atoms have more electrons that Si atoms, however that it's not a reason for an increase in conductivity.
