Average age of male = 28
Average age of female = 30
Total population = 100
So, if there are 100 people in the group, how many of them are males?
Statement 1: The average age of all 100 people is 28.9
$$\frac{Total\ age}{100}=28.9$$
Where total age = male's age + female's age
Since male's average = 28
$$Then,\ \frac{Total\ age\ of\ male}{no.\ of\ male}=28$$
Let the no. of male = x, then no. of female = 100 - x.
$$Therefore,\ \frac{male's\ age}{x}=28;\ \ \ \ \ \ male's\ age\ =\ 28x$$
$$\frac{female's\ age}{no.\ of\ female}=30;\ \ \ then\ \frac{female's\ age}{100-x}=30$$
$$female's\ age\ =\ 30\left(100-x\right)$$
Total age = (28x) + 30 (100-x)
$$\frac{\left(28x\right)+30\left(100-x\right)}{100}=28.9$$
$$28x+3000-30x=2890$$
$$2x=110$$
$$x=\frac{110}{2}=55$$
The no. of male = 55. Hence, statement 1 is SUFFICIENT.

Statement 2: There are 10 more males than females.
Total population = male + female
100 = m + f
if m = f+10, then f = m-10.
Therefore, 100 = m + m - 10
110 = 2m
m = 110/2 = 55 males
Statement 2 is also SUFFICIENT.

Since each statement alone is SUFFICIENT, the correct option is option D