Which river marked the northern boundary of islamic expansion in central asia by 750 c. e.?

The name of the river that marked the northern boundary of Islamic expansion in central Asia is Oxus. In 651, Arabs took Iran. After this, they went towards the Oxus river in the north with the intention of defeating the Turkish. The Arabs succeeded and the expansion setling Oxus river as the northern boundary was finally consolidated in 750 C.E.

the answer to your question is a. the salt plains

in 1811, he led an expedition, known as the george c. sibley expedition, to improve relations with the pawnee and kansa tribes, and also to locate the rumored jefferson's salt mountain