Hey there! I'm happy to help!
The Human Development Index takes into account 3 main things about a country:
1. Life Expectancy
2. Education
3. Standard of Living
Long work hours don't necessarily have to do with any of these categories and it can also be associated with human rights violations (slavery, sweatshops, etc.), so this will not increase a country's HDI, so A is incorrect.
A commitment to education will definitely increase a nation's HDI. Gaining knowledge is part of human development, and a great education system hugely facilitates that. B is correct.
It might seem like having the highest GDP will make a country have a better standard of living, but this isn't always the case. A country could produce tons of things but pay the workers very little, so this would not lead to a good standard living. Although a high GDP usually means that a country is doing better, it doesn't necessarily increase a nation's HDI, so C is incorrect.
D is correct because living longer means that the life expectancy is higher. This means that better healthcare, services, treatments, medicines, opportunities, etc. are available in the country, and being able to live longer is very important for human development.
Have a wonderful day and keep on learning! :D