What does writing only one word in a different colour on a poster show?

Correct answer: Emphasis through colour

Using a different colour for one word creates visual contrast within the lettering and gives that word greater attention. This is emphasis through colour, so option B is correct. The choice may also affect harmony or balance, but its immediate purpose in the question is to establish a focal word. It does not automatically mean that harmony is absent, nor does it represent proportion or balance alone. The contrast makes the selected message more noticeable.