Final answer:
The polynomial 2x(x^2-4) is factored completely.
Step-by-step explanation:
In order to determine which polynomial is factored completely, we need to look at the given options:
1. 121x^2+36y^2 - This is in the form of a sum of squares and cannot be factored further.
2. (4x+4)(x+1) - This is the factored form of a quadratic polynomial and is already completely factored.
3. 2x(x^2-4) - This can be factored further by recognizing that the term (x^2-4) is the difference of squares, resulting in 2x(x-2)(x+2).
4. 3x^4-15n^3+12n^2 - This polynomial is not factored completely and cannot be simplified any further.
Given these options, the polynomial that is factored completely is 2x(x^2-4).