Answer:
Option A (ab)(x) will form the quadratic equation.
Step-by-step explanation:
We have been given that a(x) and b(x) are linear function so we can assume
a(x)= cx+d and b(x) = ex+f substituting these values in the given options
In case A let us substitute the values a(x)= cx+d and b(x) = ex+f we will get
after simplification we will get 
This is a quadratic equation because the degree of this equation is 2.
In case B let us substitute the values a(x)= cx+d and b(x) = ex+f we will get
which is itself a simplified from and degree in this case is 1.
Hence, this is not the quadratic equation.
In case C let us substitute the values a(x)= cx+d and b(x) = ex+f we will get
after simplification we will get 
This is again not a quadratic equation since, degree in this case is 1.
In case D let us substitute the values a(x)= cx+d and b(x) = ex+f we will get
after simplification we will get 
This is again not a quadratic equation since, degree in this case is 1.
Therefore, Option A (ab)(x) will form the quadratic equation.