Translation:I am eating lunch right now.
Ditto. If someone with Chinese-grammar chops could chime in, it would be appreciated.
I don't have any concrete grammar rules to call on, but I would say I tend to use 正在 when I'm emphasising the action (kind of like saying "I'm just in the middle of eating lunch"), and 现在 more to emphasise the time (like "what are you doing?" "I'm eating lunch now, and later I'll go back to work"). That said, I would say they're pretty close to interchangeable - can anyone confirm?
I translated this as: Right now, I am eating lunch. But this was marked wrong?
I will say - "I am having my lunch now" will be more favourable.
"Right now" should be optional since it doesn't explicitly say "现在".
Duolingo seems to put too much emphasis on there being a clear distinction between the use of 在 and 正在, whereas the reality is that in many situations they are interchangeable.
If you wanted to stress that you are eating 'right now', which sounds very forceful in English, there are other structures you could use in Chinese.