This meets the criteria of this assignment yes. The requirements are just to get you to play around with editing software a little... In other words this is not great but yes you have used some post processing software to modify an image... But let me ask the question of why? The original photograph of the food is better than the edited image so what's the point?! I don't want to sound harsh or rude here but there is much more to be gained from trying to actually create something or improve something than just making a selection and desaturating the rest of the frame. Do you know what I mean?
Photoshop (and the like) can be used to really enhance your images or even create entirely new images.
The point is that it's used for a reason and not just for the sake of it.
The original photo was a perfectly decent photograph of some food and the modified image shows just some watermelon in color...
I just don't see that as an improvement in any way...
I think you know what I mean!