If the resistance of a conductor is reduced to half, its thermal effect will be . . . . . . . .

The thermal effect of current refers to the heat produced in a conductor when an electric current flows through it, and it is given by Joule’s Law of Heating.

According to Joule’s Law, the heat produced (H) in a conductor is given by the formula:

H = I² R t

where I = current, R = resistance, and t = time.

If the resistance of the conductor is reduced to half, and the potential difference (V) remains constant, the current changes according to Ohm’s law:

I = V / R

When R is halved, the current becomes double (I' = 2I).

Now, substituting in the heat equation:

H' = I'² × R' × t = (2I)² × (R/2) × t = 4I² × (R/2) × t = 2I²Rt

This means that the new heat produced is double the original heat.

Therefore, when the resistance of a conductor is reduced to half (keeping voltage constant), its thermal effect doubles.

Hence, the correct answer is Option C: double.