I answered this as 'one lettuce' but it said the correct answer is 'one head of lettuce'. Why is this? And how can you differentiate between lettuce and head of lettuce?
You probably got marked due to the fact that we don't count lettuce. The noun tends to be used collectively. It would sound odd to say, "two lettuces", unless you were talking about romaine and iceberg. There are a lot of vegetables like this--spinach, broccoli, garlic. Of course, colloquially people will pluralize all of these.
They're the same thing, and both should be accepted. Please use the report button next time you come across this question :-)
In English, a lettuce and a head of lettuce are the some thing. I reported my answer.