|Which answer below provides the best explanation for the following temperature record
||b) Natural variations in global temperatures may occur in roughly 300-years cycles actually closer to 500 years|
That is correct!
The primary cause of variations in Earth’s climate is the regular variations in the brightness of the Sun and changes in Earth's orbit about the Sun. In addition to 40-year cycles and 300-year cycles, other temperature cycles include:
19,000 year cycle: Earth's combined tilt and elliptical orbit around the Sun (‘precession of the equinoxes’).
41,000 year cycle: Cycle of the +/- 1.5 degree wobble in Earth's orbit
100,000 year cycle: Variations in the shape of Earth's elliptical orbit (‘cycle of eccentricity’)
For more information, see http://www.ngdc.noaa.gov/paleo/milankovitch.html and http://www.abc.net.au/science/news/enviro/EnviroRepublish_233658.htm