Solve it like this ;

1.In this option u put, a+b in place of x^2 , which will be (a+b)^2 is not equal to a^2+b^2
...like this 1st place a+b in place of whatever func is given and then put a in that place and b in that func and add it,then see if both a+b and a & b are equal
like in option 5, if u put f(x) = -3x, if u put -3(a+b) = -3a -3b
and if u take RHS,put f(a)= -3a & f(b)= -3b, adding these two , it will be -3a-3b which is equal to f(a+b)
So, answer is E.

For these type of problems try substituting numbers

Since square root is asked choose perfect squares for a and b(pcik easier numbers too if needed in other situations)

a= 9 b= 16

f(a+b) = f(a) + f(b)
i.e f(25) = f(9) + f(16)

f(25) = -3x = -3*25 = -75
f(9) = -3x = -3*9 = -27
f(16) = -3*16 = -48

f(25) = f(9) + f(16) TRUE

Other choices will evaluate to false and only E will be true

I should also add avoid picking 0 or 1 beacuse of the unique propoerties of these 2 numbers when doing certain operatiosn like addition/multiplication

pick other easier numbers too if needed in other situations