When parking on an **uphill slope**, it is important to take measures to prevent the vehicle from moving accidentally. One of the recommended actions is to **leave the first gear engaged** if the vehicle has a manual gearbox. This is because the first gear offers greater resistance to backward movement, helping to keep the vehicle in place.

Additionally, it is essential to **apply the parking brake** to ensure that the vehicle does not move. These combined measures provide greater safety when parking on an uphill slope.

On the other hand, if you were parking on a **downhill slope**, you should leave the reverse gear engaged, as it offers resistance to forward movement. However, in the case of an uphill slope, the first gear is the correct option to prevent the vehicle from rolling backward.