If there are boxes with $"$X$"$ in an Indexed Colour image palette, that means:

A$.$ the image has too many colours and some will have to be modified

B$.$ some of the colour shades in the image are too similar

C$.$ the image does not require as many colours as the palette can support so the image won't save properly

D$.$ the image does not require as many colours as the palette can support and this is perfectly acceptable