See my answer to your previous question. 1. the understanding of people This is correct but not very natural-sounding. 2. people' understanding This is wrong - you can't just put an apostrophe on its own at the end of this word. You can only do this with words ending in 's' (eg girls') The correct form is: people's understanding.

If you mean the 'understanding' had by, or belonging to people, the second is correct - but it requires 's'. e.g. People's understanding of many issues depends on their culture. People's knowledge/understanding of simple mathematics is often very limited.

I think the first one is correct.I saw that like that.