Which of the following should be used when you want to add a slide to an existing presentation?

Answer: Option B

When you want to add a slide to an existing presentation in PowerPoint, you should use Option B: Insert, New slide. This option allows you to insert a new slide into your presentation, maintaining the current format and layout of the slides.
File, add a new slide is not a direct option in PowerPoint for adding a slide.
File Open is used to open an existing presentation file.
File, New is used to create a completely new presentation from scratch.
Therefore, the correct answer in this context is Option B: Insert, New slide.