. Напишите отрицание и задайте во к предложению:
The sole proprietorship is the most common in many western countries.

The sole proprietorship is the most common in many western countries.

Напишите отрицание

The sole proprietorship is not the most common in many western countries.

и задайте вопросы к предложению:

The sole proprietorship is the most common in many western countries, isn't it?

What is the most common in many western countries?