Therefore, $( 3)$ and $( 4)$ are wrong. Further,

$\mathrm{B}=\frac{\mu_{0} \mathrm{Nia}^{2}}{2\left(\mathrm{a}^{2}+\mathrm{x}^{2}\right)^{3 / 2}}$

where a is the radius of the coil

At $\mathrm{x}=0, \quad \mathrm{B}=\frac{\mu_{0} \mathrm{Ni}}{2 \mathrm{a}}$

when $\mathrm{x} \rightarrow \infty, \mathrm{B} \rightarrow 0$

Slope of the graph will be $\frac{\mathrm{dB}}{\mathrm{dx}}=\frac{3 \mu_{0} \mathrm{Nia}^{2} \mathrm{x}}{2\left(\mathrm{a}^{2}+\mathrm{x}^{2}\right)^{5 / 2}}$

which means at $\mathrm{x}=0,$ slope is equal to zero or tangent to the graph at $\mathrm{x}=0$ must be parallel to $x$ - axis. Hence $(2)$ is correct and $(1)$ is wrong.