In the correction sentence "Rizo kun spicoj estas bongusta", I believe it should be bongustaj since there is a plural subject and the adjective should agree in number.
The sentence Rizo kun spicoj estas bongusta is correct, and the reason you don't say bongustaj is that bongusta qualifies Rizo that happens to be kun spicoj.
Think of it like this:
My car with two doors is blue.
Why the "s" on the second correct answer? (bongustas?)
Basically if we take an adjective like varma, and apply verb endings -i -as -is -u etc. it is a verb meaning "to be warm" so that "La pico estas varma" and "La pico varmas" are the same.
I don't understand why the verb 'estas' is apparently optional in this sentence. I got it right, but the app suggested "Rizo kun spicoj bongusta" as an alternate answer.