(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, $\mathrm{V}\propto \mathrm{I}$
Or, $\mathrm{V}=\mathit{ℝ}$,
Where $\mathrm{R}$ 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) $\mathrm{V}=\mathrm{IR};2=0.5\mathrm{R}$; or $\mathrm{R}=\frac{2}{0.5}=4\mathrm{ohm}$.