(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∝I
Or, ‌‌V=ℝ,
Where R is the constant of proportionality and is called resistance. Its SI unit is ohm.
(b) 1‌ohm is the resistance offered by a conductor of unit cross sectional area when 1A current flows through it at a voltage of 1 volt.
(c) V=IR;2=0.5R; or R=‌

2

0.5

=4‌ohm.