One Way to Measure How Long Jupiter Takes to Rotate

One way to measure how long Jupiter takes to rotate is to watch the giant storms in its atmosphere go around. A more fundamental way to measure the rotation is to:

A) send a spacecraft like the Galileo probe into the top cloud layer
B) determine how long the innermost moons take to orbit Jupiter
C) measure the changes in the planet's radio waves, which are controlled by its magnetic field
D) determine the amount of methane in the planet's atmosphere
E) you can't fool me, astronomers don't have any reliable way of knowing how long Jupiter takes the spin