Which is the completely factored form of 12x4 + 39x3 + 9x2?

Factor out the greatest common factor, 3x², from 12x⁴ + 39x³ + 9x² to get 3x²(4x² + 13x + 3). The trinomial factors as (4x + 1)(x + 3), since the cross terms add to 13x, so the completely factored form is 3x²(x + 3)(4x + 1), choice B.

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