Solution:
the sumatory of a geometric sequence is given by the following equation:
[tex]S_n=\frac{a(r^n-1)}{r-1}[/tex]In this case, we have a geometric sequence with first term 3, common ratio 3, and the number of terms 7, thus, we get:
[tex]S_7=\frac{a(r^n-1)}{r-1}=\frac{3(3^7-1)}{3-1}=3279[/tex]So that, we can conclude that the correct answer is:
[tex]3279[/tex]