To become a Judge of the High Court, one must have a certain level of experience as a practicing advocate. The correct option in this case is option 3, which states that an advocate must have practiced in the High Court for a minimum of 10 years.

Option 1, stating 20 years, is incorrect as it suggests a longer period of practice than necessary. Option 2, stating 5 years, is also incorrect as it does not meet the required level of experience. Option 4, stating 15 years, is also not the correct answer as it exceeds the minimum requirement of 10 years.

Therefore, the correct option is option 3, which correctly states that an advocate must have practiced for at least 10 years in the High Court in order to become a Judge of the High Court.