PrivateFontOptions -> {opt_1 -> val_1, opt_2 -> val_2, ...} is an option for selections that specifies settings for various font suboptions.
Properties is an option that allows specification of properties to objects and items of objects.
RadioButton[x, val] represents a radio button whose setting x is set to val when the button is clicked. RadioButton[x, val] is displayed as RadioButtonBox[0, {0}] when x is ...
RamanujanTauTheta[t] gives the Ramanujan tau theta function \[Theta](t).
Rational is the head used for rational numbers.
RootMeanSquare[list] gives the root mean square of values in list.
SimpleGraph[g] gives the underlying simple graph from the graph g.
SimpleGraphQ[g] yields True if the graph g is a simple graph and False otherwise.
SpheroidalQS[n, m, \[Gamma], z] gives the angular spheroidal function QS n, m (\[Gamma], z) of the second kind.
StateSpaceRealization is an option to StateSpaceModel that specifies its canonical representation.