"It is a good question."
Translation:È una buona domanda.
Unlike in English descriptive adjectives normally go after the noun they modify and if they are placed before their meaning change from literal to a more figurative.
Un piazza grande. = A big square. (literal meaning)
Un grande piazza. = A great square. (figurative meaning)
Una donna povera = A woman who isn't rich. (literal)
Una povera donna = A woman deserving of pity. (figurative)
Except that doesn't apply when referring to things/ideas. When used with people the meaning can change, but not in this sense with question.
It depends on whether the emphasis of the sentence is about the adjective or the noun. See this reference for explanation and examples: http://www.arnix.it/free-italian/italian-grammar/adjectives-in-italian-the-position.php
Why is this sentence translated 'È una buona domanda' rather than 'buon domanda' when previous questions were translated 'È un buon toro' and 'buon pomeriggio'? I thought buono / buona dropped its ending when it came before the noun? Or is that only for masculine nouns?
In Italian descriptive adjectives are usually placed after the noun they modify. When they are placed before the noun their meaning change from literal to a more figurative.
Una donna grande. - A big woman. (literal meaning)
Una grande donna. - A great woman. (figurative meaning)
A deeper and more thorough explanation can be found on ThoghtCo
I have updated my comment above, hopefully it is more helpful now. Please also check:
When the lesson notes tell you that buono/a/i/e can come before or after the noun and now it tells you that after the noun is incorrect, it is time to ditch Duolingo. Learning a new language is hard enough, but when what you were just taught is violated, it's time to throw in the towel.