Let's write explicitly two consecutive terms:
[tex] a_n = 3^n+4,\qquad a_{n-1} = 3^{n-1} + 4 = \frac{3^n}{3} + 4 [/tex]
This means that
[tex] 3a_{n-1} = 3^n + 12 [/tex]
which is 8 more than we expected. So,
[tex] 3a_{n-1}-8 = 3^n + 12-8 =3^n + 4 = a_n [/tex]
This is options B and C, since their statements seem the same
