What is the name of the scheme in which drivers must pay a fee in order to enter the most congested part of the city during peak hours?

The scheme in which drivers must pay a fee in order to enter the most congested part of the city during peak hours is known as "Congestion Pricing" or "Congestion Charge". Here are the steps to understand this:

1. Identify the problem: The problem here is traffic congestion in the busiest parts of a city, especially during peak hours.

2. Understand the solution: The solution proposed is to charge a fee from drivers who wish to enter these congested areas. This is done to discourage unnecessary travel and to encourage the use of public transportation, thus reducing traffic congestion.

3. Name the scheme: This scheme is known as "Congestion Pricing" or "Congestion Charge". It is used in several major cities around the world, including London, Singapore, and Stockholm.