(A) : Helium is named after sun.

(R) : Helium is available only in sun.

•  The name Helium is derived from the Greek work, 'helios' which means sun, because helium was first detected from the sun's corona.
•  It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table.

Thus Both (A) and (R) are correct. (R) does not explain (A)