Sidereal day is defined as
a.
24 hours in a day
b.
the time for one complete revolution of the earth relative to the celestial reference
c.
the time between periods when the sun is at the exact same height in the sky, for example, noon
d.
24 hours at greenwich, england

B. The time for one complete revolution of the earth relative to the celestial reference.

Explanation:

As a sidereal day is about 23 hr 56 min and 4.09 seconds so, this makes a sidereal day approx 365.24 (86,164.1) seconds. Compared to solar time which is 4 minutes longer than a sidereal day. Thus about the celestial bodies, time is taken for one complete revolution. Its is easier to point a telescope to the proper coordinate in the night sky.

radiative zone

explanation    

radiative zone is the layer of the sun which energy is transferred away from the core by radiation. although, the core is the only part of the sun that produces an appreciable amount of heat through fusion. in fact, 99% of the energy produced by the sun takes place within 24% of the sun's radius.