EASA Principles of Flight Practice Test

When a flow encounters an oblique shock, the flow is turned and the component of velocity normal to the shock is reduced, while the tangential component remains the same. This means the overall speed of the air after the shock decreases, and the local speed of sound tends to rise because the temperature increases. Put together, the Mach number after the shock is lower than the Mach number before it.

Additionally, shocks are irreversible, so total pressure decreases across the shock. That’s why the idea that total pressure behind would be higher is not correct.

So the statement that the Mach number behind is lower than in front is the correct one.