How was the rotation rate of Jupiter's core determined? -by observing the Great Red Spot
-by watching lo move across the disk of Jupiter
-by radio observations of Jupiter's rapidly spinning magnetosphere
-by measuring the oblateness of Jupiter's disk

C. by radio observations of Jupiter's rapidly spinning magnetosphere

Explanation:

Jupiter refers to one of the planetary bodies in the solar system. It is considered to be the largest (twice as large or big as every other combined) planetary body and the fifth (5th) from the Sun.

Jupiter is a planet that rotates rapidly and as a result it is referred to as being noticeably oblate. It has the fastest rotation rate among the planets of the solar system because it completes a rotation about an axis in 9.9 hours.

The rotation rate of Jupiter's core was determined by radio observations of Jupiter's rapidly spinning magnetosphere.