Select the most appropriate word to fill in the blank.

A colour blind person is not able to ______ between colours.

distinguish

The correct option to fill in the blank is distinguish.

The word "distinguish" means to be able to tell the difference between two things. It is the most appropriate word to use in this context, as it conveys the meaning of a color blind person not being able to tell the difference between different colors.

The other options are not as appropriate.

• Define means to give the meaning of something. This is not the right word to use, as it does not make sense to say that a color blind person cannot define colors.
• Observe means to pay attention to something and to take notice of it. This is not the right word to use, as it does not make sense to say that a color blind person cannot observe colors.
• Glance means to look at something quickly. This is not the right word to use, as it does not make sense to say that a color blind person cannot glance at colors.