B. (x+3)(x+5)
x^2 + 8.0 + 15 is a standard form equation (ax^2 + bx + c). When factoring standard form equations that have a coefficient of one, you want to find two numbers that add to "b" and multiply to "c". In this case, 3 + 5 equals 8.0 which is your b-value, and 3 x 5 equals 15 which is your c-value.
