Finding domain from a graph?

Domain is a property of a function, not a scatter plot. If that were the plot of a function, then fine, but it is not. Conventionally the horizontal coordinate is the independent variable, and the vertical is its image through the function. Two of the points have the same horizontal component, but different vertical components. A function cannot map the same value to two distinct values.

The domain is the set of possible *input* values (in other words, all the x-coordinates).

In your case, you can just list them all:
(2,1) and (2,5) --> 2
(4,3) --> 4
(7,2) --> 7
(9,5) --> 9
(10,10) --> 10

Answer:
a. D = {2, 4, 7, 9, 10}

Now, as a follow-up, can you figure out the *range*? That's the set of output (y) values.