Final answer:
The Channel Mixer is the correct post-production tool to fix a photograph with a slightly off white balance, as it allows for precise color channel adjustments.
Step-by-step explanation:
To fix a photograph with a slightly off white balance, the post-production tool best suited for this task would be the Channel Mixer. This tool allows for precise adjustment of color channels and can help correct the color temperature to achieve a more natural white balance. Unlike tools such as Healing Brush, which are used for spot corrections (like blemishes or dust spots), the Channel Mixer adjusts colors across the entire image. Similarly, Contrast and Exposure adjustments affect the photo's brightness and dynamic range but don't target color balance specifically.