"cheese is delicious" marked wrong. I translated all other instances of "smaczny" as delicious and it was acceptable up to this point.
As a Brit living in Poland, I would agree with Marek, "smaczny" is more like "tasty" (taste is "smak"), and "pyszny" is more like "delicious" or perhaps "really tasty".
As it's written in the comments above, I deleted those options in September. But later we decided to accept them, despite not really being the direct translation... ok, added here.
However, please note that "delicious" definitely is stronger than "tasty", so a better Polish version of your sentence would be "Ser jest pyszny".
I don't know if that's what you meant, but the only problem I see is accepting pyszny/delicious, which is more than smaczny/tasty. Deleted those options.
The adjective has to match the noun it describes.
"smaczny" is masculine. "smaczna" is feminine. "smaczne" is neuter (and 'not masculine-personal plural').
"ser" is masculine, so the right form is "smaczny".
Look at the basic, Nominative form of a singular noun.
If it ends with a consonant, it's probably masculine.
If it ends with -a, it's probably feminine.
If it ends with -e, -ę, -o or -um, it's neuter.
There are exceptions to that (e.g. 'mysz' is feminine, 'mężczyzna' or 'tata' are masculine), but the above can be your first thought, and then you learn the exceptions.