The space inside a straight current carrying solenoid is filled with a magnetic material having magnetic…

Given here, $\chi =1.2\times {10}^{-5}$

Magnetic field inside the material in solenoid is $B\text{'}={\mu }_0\left(1+\chi \right)ni$, here, $i$ is current.

Now, magnetic field with air as medium inside solenoid is $B={\mu }_{0}ni$.

So, fractional increase is $\frac{B\text{'}-B}{B}=\chi =1.2\times {10}^{-5}$