Answer:
The test charge will take the south-west direction indicated in option 6.
Explanation:
The image is shown below.
Since all the charges are positively charged, they will all repel each other. If we consider the force on +q due to +Q and +Q, then we can proceed as follows
The +Q particle at the top left corner of the cube will exert a vertical downward force on +q in the -ve y-axis.
The +Q particle at the bottom right corner of the cube will exert a force on +q towards the horizontal left on the -ve x-axis.
Both of these forces will act at angle of 90°, and therefore, the resultant force will act at an angle of 45° to horizontal and vertical forces.
The result is that the +q charge will move in a south-west direction of the cube.
