The answer is C.
Remember FOIL: First terms, outside terms, inside terms, last terms.
The first terms in each of the parentheses are 3x and 4x^2. Multiply them together. 3x × 4x^2 is the first part.
The outside terms in each of the parentheses are 3x and 5. Multiply them together. 3x * 5 is the second part, which eliminates A, as that has no 3x * 5.
The inside terms in each of the parentheses are -1 and 4x^2. Multiply them together to get -1 * 4x^2. This eliminates B as it states that the third multiplication is 1 * 4x^2.
Therefore, the correct expansion is Option C.