The space inside a straight current carrying solenoid is filled...

Given here, $χ = 1 . 2 \times {10}^{- 5}$

Magnetic field inside the material in solenoid is $B ' = {\mu}_{0}\left(1 + χ\right)ni$, here, $i$ is current.

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

So, fractional increase is $\dfrac{B ' - B}{B} = χ = 1 . 2 \times {10}^{- 5}$