The correct answer is Sindhu. Let`s look at all the options for clarity:

Option 1, Sindhu, today known as the Indus River. The early Vedic literature, particularly the Rigveda, mentions the Sindhu River more often than any other rivers. It`s considered one of the most sacred rivers in the Vedic era. Hence, Sindhu is the correct answer.

Option 2 is Sutudri, now known as the Sutlej River. While it is mentioned in Vedic texts, it is not the most mentioned river.

Option 3 is Sarasvati, often discussed and indeed revered in Vedic literature. Though highly significant, it does not surpass Sindhu in frequency of mentions.

Option 4, Ganga, revered as the holiest river in India, surprisingly does not have many mentions in early Vedic literature.

Thus, going by frequency of mentions in early Vedic literature, Sindhu is the clear choice.