Answer: Choice D) 23x^9 - 12x^4 + 19
A single variable polynomial is in standard form when the exponents are decreasing as you move from left to right. The exponents 9, 4, 0 go downhill so to speak.
The term 23x^9 is the leading term with 23 as the leading coefficient. The degree of this polynomial is 9 which is the largest exponent. This polynomial is a trinomial as there are 3 terms here.