Interference was observed in an interference chamber when air was...

When chamber is evacuated, the refractive index ( $\mu$ ) decreases. Therefore, wavelength $\left(\lambda \propto \frac{1}{\mu}\right)$ increases. Now, fringe width is given by $$\begin{aligned} & \beta=\frac{\lambda D}{d} \\ \Rightarrow \quad & \beta \propto \lambda\end{aligned}$$ Therefore, as wavelength increases, the fringe width will increase.