In his letter to Louisa Bethley, Milo Thompson promised to:" O love her until she" died. O helped her escape from the plantation on which she was a slave. Marrying her at her church on the following Saturday.

Based on the choices given, it seems you might be referring to a specific historical document or a piece of fictional literature where Milo Thompson is a character who has written a letter to Louisa Bethley. Without the specific context or the text of the letter, it's impossible to know which promise Milo Thompson made to Louisa Bethley . However, you've provided three promises to choose from:

1. Milo promised to love her until she died.

2. Milo promised to help her escape from the plantation on which she was a slave.

3. Milo promised to marry her at her church on the following Saturday.

Each option carries with it a different implication and would suit a different narrative or historical context. Without more information, I could not determine which promise was made. If you could provide the actual text of the letter or more context around it, it would be possible to give a more accurate answer.