There are many ways to learn to code these days. From coding bootcamps like Flatiron School to online courses to totally free beginner resources. The best way to learn to code depends on many factors, but the most important one is you. See what course is best for you.
A case can be made for both B and D
B does not connect the points with straight lines as does D (Book appears to want st lines)
Both show the curve outside the domain which ends at 25.
The function is that of a parabola so B is functionally correct.