How did Copernicus eventually solve the problem of "retrograde motion"?

How did Copernicus eventually solve the problem of "retrograde motion"--the apparently irregular motion of the planets as they appear against the field of stars?

a. By re-envisioning planetary orbits as circles, instead of ellipses.

b. By re-envisioning planetary orbits as ellipses, instead of circles.

c. By placing the earth at the center of his model of the universe.

d. By removing the earth from the center of his model of the universe.How did Copernicus eventually solve the problem of "retrograde motion"?The answer is D, by having the earth orbiting the sun, the retrograde motion was resolved as retrograde motion was found to result from the Earth catching up with an outer planets orbit then overtaking it here's the textbook explaination

"Since Earth's orbital period is shorter than that of a superior planet, the Earth overtakes a superior planet and passes it. This occurs while the planet's configuration changes from western quadrature through opposition to eastern quadrature. During this period of passing, the planet appears to temporarily interrupt its normal eastward motion relative to the stars and move westward. This countermotion is known as retrograde motion, in which the superior planet executes a closed or open loop and then continues its usual path eastward relative to the stars. Relative to the Earth-Sun line, however, it is moving toward an area of the sky east of the Sun"

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