C) When is digital manipulation not acceptable?

It's unacceptable when cyber companies make kiddy pron (blogger won't allow the real word) by manipulating images of young girls into children. I'm sure there are many instances where digital manipulation would be frowned on, like subliminal messaging, propaganda, etc.

B) When is digital manipulation acceptable?

Digital manipulation is acceptable for in many instances,
especially for photo restorations and and restoring hostorical or ancient paintings. It's also very useful in the creation of marketing and promotion pieces for business purposes.

Is it easy to tell when an image has been digitally enhanced? What are some clues?

I wouldn't necessarily say it's easy or challenging to decipher a digitally enchanced image. It really depends on the software you are using to test if an image has been digitally enchanced or of things are noticeably out of place. For example a woman's face on a man. Though that might not too far stretched in this day and age, but you get the jest of it.

Some clues that you can look for are:

1. Does the scenery exist?
2. Are all elements of the image in proportion?
3. Do colors match and look like they are in place?
4. Is what you're looking at possible?
5. Do you really think that's what Jennifer Aniston looks like naked? LoL.