input: Write a function to create a basic REST API using Python and Flask to list all the countries in the world. The function should return the Flask application instance.
  import flask
  from flask import Flask, request, jsonify
  import json
  
  def create_country_api():
      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)
  
      if name == 'main':
          app.run()
  
      return app
  
  assert isinstance(create_country_api(), Flask) == True
output: 
  def create_country_api():
      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
  