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The answer is either '7 units' or I would accept just '7' for that is what it is.
It's unusual to have a question like this in isolation. Normally it would be of the form:
What is the value of the 7 in the numbers a) 764 b) 8375 c) 72 d) 17 e) 7 892? (Difficulty of numbers depending on age and ability, of course. So '7' as the answer to parts d) and e) would be perfectly acceptable and would show that the concept has been understood, provided the others are correct. By the way, the answers could be written in digits or words, so the following answers are all equivalent and correct for part a) for example:
'700' '7 hundred' 'seven hundred'
Later there will be decimals such as 8.57 and 0.734 and the answer to '8.57' could be written as 0.07, 7 hundredths, seven hundredths or 7/100