Which two countries make up the coastline of the riviera?

France and Italy (plus Monaco)

Explanation:

The riviera is a very attractive coastal area on the Mediterranean Sea. It is stretching in the southeastern coastal part of France, and the northwestern coastal part of Italy, with the micro country Monaco also having a very small share in it. This region is regarded as one of the most attractive in Europe. There's long beautiful beaches, warm waters, the Alps lifting up north of it, lot of luxurious resorts and hotels, nice architecture, old towns and cities that have been well preserved, all in all making it an excellent tourist destination. Some of  most attractive the cities that are lying on the riviera are Marseilles, Toulon, Nice, Monaco, and Genoa.