Translation:Today's dinner is vegetables.
You will soon start to see that there are mor synonyms than you could ever need. It's due to the massive amount of borrowing from Chinese that occured off and on throught history. ばんごはん is Chinese in origin. We can tell this because of the honorific GO applies to chinese nounds whrre-as o applies to Japanese nounds and modern borrowings from english like rinbon is oribbon.
Some people think that because in the morning, 朝 (asa), we eat 朝ご飯, and in the afternoon, 昼 (hiru), we eat 昼ご飯, then at night, 夜 (yoru), we ought to be eating 夜ご飯. However, the word 夜ご飯 didn't initially exist in Japanese. Someone just made that word up recently. So the term 夕飯 (yuuhan) is generally used for 'dinner'.
Not really, no. There are some speakers who might use that, but basically no. Dinner is the main meal of the day, in the (traditionally late) evening. Lunch is the midday snack / meal, and there are various others whose name / timing varies according to location and social class. This Japanese sentence is specifically an evening meal, so again, not lunch.
I've got the audio version of this exercise and I've typed in 「今日の夕飯は野菜です。」 However, not only it was marked incorrect, I can't even report that it should also be accepted.
It's getting incredibly frustrating to use the keyboard and rely on their puzzle-like input system, or try to input everything in kana.