Option A: ggplot(diamonds, aes(x= carat, y=price, color=color))+geom_point()

The first line of code assigns color to represent the color of the diamond so we would see 5 different colored points associated with each of the different diamond colors, as shown below.

Option B: ggplot(diamonds, aes(x= carat, y=price, color=color))+geom_point(color="light blue")

This is the correct answer. Although we specified the color in the aes function to represent the color of the diamond we also specified the color in the geom to be blue. Since the color 'light blue' in the geom layer is specified after the aes mapping of the color it overrides it. The image is shown below.

Option C: ggplot(diamonds, aes(x= carat, y=price, color="light blue"))+geom_point()

The third line of code assigns the color in the aes function to be "light blue". Light blue is not a variable in the diamonds data set so you are basically asking R to map a non-existent variable called "light blue" on to the plot. The result is shown below.

Option D: ggplot(diamonds, aes(x= carat, y=price), color="light blue")+geom_point( aes(color=color))

Here, we cannot make the points blue by specifying the color outside of the aes function, in the ggplot function. This will just produce a plot with black dots, which is the default. But because we specified the aes mapping of the color in the geom function to be the color of the diamond, the graph ultimately looks like the first one. The result is shown below.