(a) According to Ohm's law, the current flowing through a given conductor is directly proportional to the potential difference along its ends provided the the temperature and physical conditions remain the same.Mathematically, $V \propto I$Or, $V = \mathbb{R}$,Where $R$ is the constant of proportionality and is called resistance. Its SI unit is ohm.(b) $1 \mathrm{ohm}$ is the resistance offered by a conductor of unit cross sectional area when $1 \mathrm{A}$ current flows through it at a voltage of 1 volt. (c) $V = IR ; 2 = 0.5 R$; or $R = \frac{2}{0.5} = 4 \mathrm{ohm}$.