Step 1: Examine the assertion.
Ethylene is the only naturally occurring gaseous plant hormone.
Hence,
\[
\boxed{\text{Assertion [A] is true}.}
\]
Step 2: Examine the reason.
Ethylene does not inhibit fruit ripening.
Instead, it promotes fruit ripening, senescence and abscission.
Therefore,
\[
\boxed{\text{Reason [R] is false}.}
\]
Thus,
\[
\boxed{(C)}
\]
is the correct answer.