Learn English – “Something is off”

Off here indicates not the location of living but its source.

The idiom arises in the last half of the 18th century in association with the idiom “get one’s living off (of) the land”, meaning one obtains the income which sustained life by farming or by renting the farmlands one owned to others. Armies were likewise said to “live off the land” or “off the country”—meaning that in wartime they obtained what they needed by appropriating it from the territories they crossed rather than being supplied from home.

A similar use of off appears in the phrase dine off (pheasant, the haunch, six or seven courses) meaning one obtains one’dinner from these sources.

So to say that “She lives off her parents” means she obtains the necessities of life from her parents; to say that “I've been living off rats” means I get my nutrition by eating rats.

These jeans are 25 percent off.

is even more idiomatic. There's nothing wrong with d) or e). A small change to b) or c) gives:

These jeans have a 25% discount.

which is a little more natural than d) or e).