X-rays from a Coolidge tube are incident on a thin aluminium foil,...

Increasing the heating current increases the temperature of the filament in the Coolidge tube.

This leads to an increase in the rate of thermionic emission, causing more electrons to strike the target per second.

As a result, the number of X-ray photons produced per second increases, which means the intensity of the incident X-rays increases.

Since the accelerating voltage across the tube is unchanged, the maximum energy of the X-ray photons and the absorption coefficient of the aluminium foil remain constant.

The transmitted intensity is directly proportional to the incident intensity given by $I = I_{incident} e^{-\mu x}$.

Since the incident intensity increases while the fraction of transmission remains constant, the new transmitted intensity will be greater than $I_0$.