A good multiple choice grammar quiz/exam question should have exactly one correct answer and one, two or three incorrect answers. The other choices should be plausible enough for a student to have to think about it, but not an alternative correct (but lesser used/dependent on context) answer.
Last week, my students had their final exam for this semester. I compiled it from the question bank provided by the publisher. It wasn’t until I was marking one of the exams that I noticed that two questions had alternative correct answers. In each case there was an expected answer, but one of the other choices was not, by any standard, incorrect.
The grammar point was countable and uncountable nouns. The questions were:
 Carlos eats ____ apple every day.
 Did you eat ____ ice cream for dessert?
In , apples start by being countable – one apple, two apples etc, and the expected answer is ‘an’. But many foods can be chopped, grated, pureed etc to become uncountable. Carlos might be a baby just starting on solids, in which case he could very easily eat ‘some apple’ (but less than ‘an apple’) every day. He may be older, but just not capable of eating or inclined to eat a whole apple every day. (Some of the apples available in South Korea are enormous; I couldn’t eat a whole one every day even across breakfast, lunch and dinner.) Google Ngram Viewer backs me up on this. ‘An apple’ is, of course, far more commonly used, but ‘some apple’ shows up – it’s not ‘incorrect’.
In , ice cream for dessert starts by being uncountable – it’s just ice cream, and the expected answer is ‘some’, maybe one scoop, maybe more, maybe a bowl of gloopy liquid by the time you’ve finished stirring it. But ice cream also comes in discrete blocks on a wooden stick – you can buy and eat ‘an ice cream’, for dessert or any other time.
Intriguingly, Google Ngram Viewer shows that by itself ‘an ice cream/ice-cream/icecream’ is far more commonly used than ‘some ice cream/ice-cream/icecream’. Many people buy and eat ‘an ice cream’ at all times of the day. On the other hand, there are (very few) results for ‘some ice cream for dessert’, but none at all for ‘an ice cream for dessert’. But in the days of tv dinners, ‘an ice cream for dessert’ is not ‘incorrect’.
Most students gave the ‘correct’ answer anyway. Maybe one or two gave the ‘incorrect’ one. Maybe I worry too much.
(By the way, ice cream, ice-cream and icecream are the spellings in that order. Google Ngram Viewer records icecream, but the spell-checkers on Pages for Mac and WordPress don’t like it.)