Which of the following monitors has only two colours

Computer monitors are classified partly by how many colours they can display.

A monochrome monitor displays images using only one foreground colour against a background, which in practice means the screen shows just two distinct colours, for example black and white, or green and black. This limitation comes from the display technology only being able to represent a pixel as either on or off, with no shades or additional colours in between.

The other options do not fit this description. A colour monitor displays a wide range of colours by combining red, green, and blue light in varying intensities. Colour Graphics Adapter (CGA) was an early IBM display standard capable of showing several colours at once (up to 16 in text mode or 4 on screen simultaneously in graphics mode), which is more than two. "Dichrome monitor" is not a standard recognised term in computer studies; the correct technical term for a two-colour display is monochrome.

When a question describes a display limited to exactly two colours, associate that with the term monochrome, since colour monitors and CGA both support more than two colours at once.