Which river was regarded as the most sacred river of the Rigvedic period and called ‘Naditama’?

The Rigveda praises Saraswati as Naditama, the best or foremost of rivers. The Indus was also a major river, while the religious prominence of the Ganga and Yamuna increased especially in later periods. Saraswati fits that evidence because it matches the specific chronology, agency and function contained in the question. A comparison with the alternatives is useful: Ganga, Yamuna and Indus are genuine historical terms or names, but each belongs to a different office, patron, text, region or sequence. The common error is to select the most familiar option without checking whether it performs the role described in the stem. Reading the question as a relationship among person, period, place and institutional purpose prevents that mistake. The compact memory link is therefore Saraswati together with the factual context above, not the isolated option letter.