"This lunch is delicious."
Pretty much. There are other ways to say it too but that's generally the most used one.
Why does it need to specifically be "this lunch" instead of just "lunch". Wouldn't the "this" be implied?
Duolinguo can sometimes be annoyingly inconsistent.
In part of this lesson it asks you to translate "This menu" and if you use Kono you are marked wrong.
Yet when it asks you to refer to "This lunch" you are marked wrong for not using Kono.
Make up your mind little green owl =P
この ひるごはん は おいしい is marked as correct, I guess the です is only to make it more formal.
What are the conditions under which I'd include or drop the を in front of ひるごはん? As I understand, it's to do with politeness, but as it's a part of speech not dealing with a person or basics like please/thanks, are recommendations for including this を a bit more lax than stuff like pronoun choice?
There is no object in this sentence so it cannot include any を.
Edit: Sorry I think you mean お. It is optional here and does not relate to any politeness. It is customary お.
Breakdown: このお昼ご飯は(kono ohirugohan ha, This [applies to next word]+lunch+topic marker) おいしいです(oishii desu, good+is).