the answer is (A)

x^4 + y^4 > z^4 - is this true ?

statement 1
x^2 + y^2 > z

lets plug in:

(1) x,y = 1/2 z = 2

or alternatively

(2) x,y = 2 z = 2

I'm only useing negative numbers since n^2 or n^4 will give a positive outcome.

for (1):
1/4+1/4 > 4 (this is false)

for (2):
4+4 > 4 (this is true)

we will applay this to the stem (x^4 + y^4 > z^4)

to prove it's correctness !

sufficient

statement 1
lets plug in:

(1) x,y = 2/3 z = 1

or alternatively

(2) x,y = 2 z = 2

for (1):
2/3+2/3 > 1 (this is true)

for (2):
2+2 > 2 (this is true)

if we will applay (1) & (2) to the stem since they are both true (x^4 + y^4 > z^4)

we will once get true (2) and once false (1)

insufficient

hence the answer is (A)