When adding two different signed numbers, the absolute values of the numbers and keep the sign of the number with the larger absolute value

Option C is correct.

Step-by-step explanation:

The given equation or function is linear if the variables x and y have degree zero or 1.

The given equation is 5x^4+5y =9

here x has degree =4 and y has degree = 1, but for linear equation both x and y variables must have degree zero or 1.

So, Option C is correct.