C. 2916 Mice
Explanation:
4 x 3^6 = 2916
Because as the mice increases, you triple that amount, instead of tripling 4 & multiplying by 6.
So, you're doing this instead:
4 x 3 = 12 (1 wk)
12 x 3 = 36 (2 wk)
36 x 3 =108 (3 wk)
108 x 3 = 324 (4 wk)
324 x 3 = 972 (5 wk)
972 x 3 = 2916 (6 wk)
Hope this helps!