B.
The minus sign applies to all terms in the right hand parenthesis. So the x^4 and x^2 drop out. 2x^3+2x remains.
If we rewrite the equation without parenthesis, apply the sign in front of the parenthesis to all terms like so:
x⁴+x³+x²+x - x⁴ - -x³ - x² - -x
Now, realizing that - - = +, you can see that they flip sign and you get:
x³+x³+x+x = 2x³ + 2x
(I already removed x⁴-x⁴=0 and x²-x²=0)
