The first and third selections are infinite sums. The given sum is finite, so those choices are incorrect.
The second choice fails to give the correct value for n=0. (It gives 0, not -3.)
Thus, all choices are eliminated except the last one. It gives the summation of the correct terms for the correct number of terms.
The 4th selection is correct: [tex] \sum\limits^{15}_{n=0}{(-3+9n)}[/tex]
