To preview a motion path effect using the custom animation task pane, you should click the play button.
The play button is located in the lower-right corner of the custom animation task pane. When you click the play button, PowerPoint will play the animation in the presentation window. You can also use the play button to stop, pause, and rewind the animation.
The show effect button is located in the lower-left corner of the custom animation task pane. When you click the show effect button, PowerPoint will show the animation in the presentation window without playing it. This can be helpful if you want to see how the animation will look without actually playing it.
Double clicking the motion path will not preview the animation. It will open the Format Motion Path dialog box, where you can customize the motion path.
Therefore, the correct answer is A.