input: from typing import List, Dict
  import flask
  from flask import Flask, jsonify
  
  def create_countries_api() -> Flask:
      """
      Creates a basic REST API using Python and Flask to list all the countries in the world from a JSON file.
      The API has a single endpoint '/countries' which returns a list of countries in JSON format.
      
      >>> app = create_countries_api()
      >>> client = app.test_client()
      >>> response = client.get('/countries')
      >>> response.status_code
      200
      >>> isinstance(response.json, Dict)
      True
      """
      app = Flask(__name__)
  
      @app.route('/countries', methods=['GET'])
      def get_countries():
          with open('data.json', 'r') as f:
              countries = json.load(f)
          return jsonify(countries)
  
      return app
  
  if __name__ == '__main__':
      app.run()
output:     
      import json
  
      return app
  