Which is the S.I unit of resistance?
A. ampere
B. mho
C. ohm
D. farad

Resistance is basically defined in Ohm’s law as a constant. According to Ohm’s law current (I) through the conductor is directly proportional to voltage difference (V) between two points. We can write it as:
$V\propto I$
When we remove the proportional sign we will insert a constant which is resistance (R).
$V=IR$
On the basis of the name of this law unit of resistance is defined in the S.I unit.

Complete step-by-step answer:
According to Ohm’s law we have equation$V=IR$.
We can write it as
\[R=\dfrac{V}{I}\]
So we can say resistance is defined as the ratio of Voltage difference and current passing through the conductor. We can also say resistance of a conductor is an opposition offered by conductor when electric current passes through it. So we can say if there is less current passing through the conductor then resistance of that conductor will be high.
S.I stands for international system. In this unit of resistance is ohm. We represent it via the symbol omega $(\Omega )$.
Hence option C is correct.

Note: As we define R is the ratio of voltage difference to current. So we can say the unit of resistance is also defined as volt per ampere.
So we can write
$1\,ohm\,=\,1\,volt\,per\,ampere$