Unit Vector means the modulus should be 1. Let the Vector be X.

Hence, |X|= (0.4*0.4+0.8*0.8+c*c)^0.5

Where |X|=1

Hence on Squaring Both Sides and Solving we get

c= (0.2)^0.5

It means Component of the Vector in z direction, extends a distance of square root of 0.2 .

If the angle made by the Unit Vector X and the z axis is ×, then |X| Cos× is square root of 0.2 .