In Buddhist philosophy, the practice of Si la refers to mental control. It involves disciplining and training the mind to cultivate virtuous behavior, ethical conduct, and mindfulness. Si la is an essential aspect of Buddhist practice as it helps cultivate mental stability, clarity, and inner peace. By practicing Si la, individuals gain control over their thoughts, emotions, and conduct, and develop a strong foundation for further spiritual growth.

Option 1: Mental control - This option is correct. Si la primarily focuses on mental control and discipline.

Option 2: Physical control - This option is not accurate. While physical control may be a part of some Buddhist practices, it is not specifically referred to as Si la.

Option 3: Development of insight - Although the development of insight is important in Buddhism, it is not directly related to Si la.

Option 4: Strict penance - While penance may be practiced in some spiritual traditions, it is not the primary focus of Si la in Buddhism.

In summary, the correct answer is option 1 - Mental control.