input: 
  from flask import Flask, jsonify
  import json
  
  app = Flask(__name__)
  
  @app.route('/countries', methods=['GET'])
  def get_countries():
      """
      Create a basic REST API using Python and Flask to list all the countries in the world.
      >>> app.test_client().get('/countries').status_code
      200  # Assuming 'data.json' contains valid country data
      """
      with open('data.json', 'r') as f:
          countries = json.load(f)
      return jsonify(countries)
  
  if __name__ == '__main__':
      app.run()
  
output:     
      with open('data.json', 'r') as f:
          countries = json.load(f)
      return jsonify(countries)
  