The theme of the World Tourism Day in 2019 was "Tourism and jobs: a better future for all". This theme highlights the importance of tourism in creating employment opportunities and improving the lives of individuals around the world.

Option 1: Sustainable tourism - While sustainable tourism is an important aspect of the tourism industry, it was not the specific theme of World Tourism Day in 2019. Sustainable tourism focuses on minimizing the negative impact of tourism on the environment and promoting responsible tourism practices.

Option 2: Tourism and the digital transformation - This option is not the correct answer. While the digital transformation has had a significant impact on the tourism industry, it was not the theme of World Tourism Day in 2019.

Option 3: Tourism: responding to the challenge of climate change - This option is not the correct answer. Climate change and its impact on tourism is an important issue, but it was not the theme of World Tourism Day in 2019.

In conclusion, the correct answer is option 4: Tourism and jobs: a better future for all. This theme focuses on the role of tourism in generating employment opportunities and creating a better future for individuals.