The correct answer is (B):
Let the average age of people aged 51 years and above be x years.
Let the average age of people aged below 51 years be y years.
Let the number of people aged below 51 years be N.
Given, the average age of all the people in the apartment complex is 38 years.
Therefore,
x×30+y×N/30+N = 38...(1)
We want to maximize y, which occurs when x is minimum i.e. for x=51.
Substituting the value of x in (1) we get
390=N×(38-y)
Again, when y is maximum, N is also maximum i.e. 39
Therefore maximum value of y = 28.