To prevent backscatter when taking underwater photographs, the flash should be:

To prevent backscatter when taking underwater photographs, it is essential to hold the flash as far away from the camera as possible. Backscatter occurs when particles in the water reflect the light from the flash back towards the lens, resulting in undesirable bright spots or hazy images. By positioning the flash farther away from the camera, you can reduce the likelihood of the flash illuminating these particles, which helps to capture clearer and more vibrant images of the subject.

This technique effectively allows for more direct lighting on the subject while minimizing the impact of the suspended particles in the water column. The other answer options do not address the issue of particle illumination as effectively as this method. For example, setting the flash to automatic may not adjust adequately for the underwater conditions, while using the flash only when the subject is above water defeats the purpose of underwater photography. Mounting the flash on the camera can exacerbate backscatter because it keeps the light source close to the lens, making it more prone to reflecting off particles in the water.