If the first term of an A.P. is a and nth term is b, then its common difference is

Answer: Option B

In the given A.P.
First term = a and n^th term = b
$$\begin{array}{rl}& \therefore a+\left(n-1\right)d=b\\ & \Rightarrow \left(n-1\right)d=b-a\\ & \Rightarrow d=\frac{b-a}{n-1}\end{array}$$