Answer:
A.The positive z-direction
Step-by-step explanation:
We are given that 
Linear charge density of long line which is located on the x-axis=

Linear charge density of another long line which is located on the y-axis=

We have to find the direction of electric field at z=a on the positive z-axis if 
 and 
 are positive.
The direction of electric field at z=a on the positive z-axis is positive z-direction .
Because 
 and 
 are positive and the electric field is applied away from the positive charge.
Hence, option A is true.
A.The positive z-direction