For a first-order reaction, the time taken for the concentration of a reactant to reduce to 90% of its initial value is related to the time taken for the concentration to reduce to 99%. The relationship is derived from the integrated rate law for a first-order reaction.
\[
t_{99} = 20.693 \times t_{90}
\]
Thus, the correct relation between \( t_{90} \) and \( t_{99} \) is:
\[
t_{99} = 20.693t_{90}
\]
Final Answer: (D) \( t_{99} = 20.693t_{90} \)