Resistor is heated due to the current flowing through it. Thermal energy is increase.

Formula used :

Where i→ current

A→ area of the cross section

n→ electrons per unit area.

→ Drift speed

R→ resistance

ρ→ resistivity

l→ length

A→ area of cross section

If thermal energy is increase the resistance will increase. Resistance is directly proportional to resistivity, so resistivity also increase. Increase in resistance leads to decrease in current. Current is directly proportional to drift velocity. So drift speed. So only the number of electrons remains same.

If the resistor connected to a battery is heated due to current flowing through it the drift speed, resistance, resistivity all are change except number of free electrons. Option D is correct.