Answer: C
Explanation:
Standard form for multi-variable expession( quadratic, trinomial, polynomial, etc) are represented by something like this.
ax^2+ bx+c, which means the stating term is the degree or exponet with the highest number then as we go right the degree go smaller.
So that means 5x^4 will be the starting term so a and b are ruled out. The next term will be x squared o x^2 so the only answer that has it second is C