The holy Hindu scriptures written by gurus to help connect people to heavenly forces include:
- the Upanishads
- the Mahabharata
- the Ramayana
However, the "Theravada" refers to one of the branches of Buddhism and is not a Hindu scripture.
So, the correct responses from your list are:
- the Upanishads
- the Mahabharata
- the Ramayana
If you need to select one, the Upanishads are specifically noted for their philosophical content relating to the connection to Brahman (the ultimate reality).