Which of the following statements is not true?

The correct answer is Option (3) → Semiconductors obey Ohm's law.

Ohm's law (\( V \propto I \), with a constant resistance) holds only for ohmic conductors under steady physical conditions. Devices whose V-I graph is not a straight line are non-ohmic.

Option (1) Conductance is the reciprocal of resistance. — True statement. Conductance is \( G = 1/R \); its unit is the siemens (S).

Option (2) Ohm's law is not applicable at very low and very high temperatures for conductors. — True statement. Ohm's law needs the resistance to stay constant. At very low temperatures some conductors become superconducting, and at very high temperatures the resistance changes strongly, so the simple linear relation fails.

Option (3) Semiconductors obey Ohm's law. — Not true. Semiconductors are non-ohmic: their V-I graph is not linear (a diode is the clearest example), and their resistance falls as the temperature rises. This is the statement asked for.

Option (4) Ohm's law is not applicable to electron tubes, discharge tubes and electrolytes. — True statement. Electron tubes, discharge tubes and electrolytes have non-linear V-I characteristics, so they do not obey Ohm's law.

NCERT (2026-27):
Limitations of Ohm’s Law – “Although Ohm’s law has been found valid over a large class of materials, there do exist materials and devices used in electric circuits where the proportionality of V and I does not hold. … (b) The relation between V and I depends on the sign of V. … This happens, for example, in a diode which we will study in Chapter 14. (c) The relation between V and I is not unique, i.e., there is more than one value of V for the same current I (Fig. 3.7). A material exhibiting such behaviour is GaAs.”