Why is the answer d? plz explain i’m so confused

X=estimated time
y=actual time

What is needed is the actual time be within 10 hours of the estimated time.

That means that if project is finished within 10 hours before the estimated time (i.e. x>y), then
x-y<10 which is equivalent to -10<y-x    (see * below)
And if project is finished within 10 hours after the estimated time (then y>x)
y-x<10

Combine the two conditions, because the estimate can be over-estimate, or underestimate,
we have
-10<y-x  and y-x < 10
or, written in a single line, 
-10 < y-x < 10

*Note:
x-y<10
subtract 10 from both sides
-10+x-y < 10-10 => -10+x-y < 0
now subtract x-y on both sides
-10+x-y-(x-y) < -(x-y)    distribute the parentheses
-10+x-y-x+y < -x+y   simplify
-10 < y-x
So  x-y <10  is the same as -10 < y-x

Remark
What the question is actually saying is that the time to be taken is 100 +/- 10. That is what you might thing at first glance. Actually it is supposed to be a bit over 100 hours. 

So let's start with the letters.
y, according to the problem, is the actual time it takes to complete the project.
y takes into account that the actual project could take 100 or so hours.
The tricky part is what is x?
That value is an estimate of how long the project will take, also near 100.

So  y- x is ideally +/- 10 meaning the estimate and the actual should not differ by more than 10 hours in either direction. If it is  under -10 then  the customer is getting ripped off, (because more time should have been taken than was to do the project). If it is greater than +10 then the company is getting ripped off because the time to do the project was an over run. 

So the ideal situation is 
-10 <actual time taken - theoretical time estimated < 10 or
-10 <y - x <10