If you cannot switch to a faster film to achieve proper exposure, which option could help using existing light?

Using a tripod or flash allows you to utilize existing light more effectively while maintaining a suitable exposure. When using a tripod, you can stabilize your camera and reduce the risk of camera shake during longer exposure times, which means you can use a slower shutter speed without blurring the image. This also provides flexibility to use smaller apertures, which can increase the depth of field and enhance the overall sharpness of the image.

Additionally, adding flash can illuminate the scene and allow you to adjust the exposure settings, providing adequate light to capture a properly exposed photograph even in low-light conditions. This method lets you maintain control over the aperture and shutter speed settings while leveraging available light sources effectively.

Adjusting the ISO settings could also be a solution, but this option specifically relates to changing existing settings rather than utilizing current conditions effectively. Adjusting white balance primarily affects the color temperature, and lowering the aperture to a very wide setting like f/1.4 would increase exposure but may lead to a shallow depth of field and potential focus issues in certain conditions. Thus, the combination of a tripod or flash is the most effective strategy among the choices provided for achieving proper exposure with existing light.