CMYK are the primary colours of pigment (well, Cyan, Magenta and Yellow are - the black is a key colour, hence the “K”). But you may find yourself editing some artwork in Illustrator and you need to rasterize some vectors and/or images and the only option Illustrator gives to you is CMYK. This is fine if you are going to print the artwork because you will need the image in the primary colours of pigment in order to print them, but if you only want to view the artwork on screen or make a PDF for on screen viewing or if you are creating a web image, then you will want to rasterize as RGB.
How DO you rasterise images or objects in Illustrator as RGB if there is no option to do so?
Actually there IS a rasterise as RGB option in illustrator and it’s easy to find.
The reason you are only getting the CMYK option when rasterising is because you are working on a document set in the CMYK mode. The CMYK mode is available when you create a new document under the advanced area of the new document dialogue (as shown:)
All you have to do to get the option to rasterise as RGB under objects>rasterize is change the colour mode of the document. You can do this via file>document colour mode, there you can choose between CMYK or RGB.
Illustrator will only offer you the ability to resterize in the same colour mode of the document, so after you change the document color mode to RGB, the RGB rasterize now be available under objects>rasterise.