Select the appropriate preposition to fill in the blank in the following…

Concept: The preposition "in" is used to show a static location enclosed within, or forming part of, a larger area or region. With the noun "source" (the place where a river originates), the fixed idiomatic collocation in standard English is "has its source in [a place/region]" — describing the river's origin as lying within that region.

Application: The Brahmaputra originates from within the Himalayan mountain range — its source lies inside this vast region. The sentence therefore completes correctly as "The Brahmaputra has its source in the Himalayas," using "in" to show that the source is located within that region.

Contrast: Compare the other prepositions and the different meaning each expresses here:

• "into" expresses movement or entry from outside to the inside of something (e.g., "walked into the room", "poured water into the glass") — it pairs with verbs of motion, not a static existing location.

• "under" expresses a position vertically beneath or below another object or surface (e.g., "the ball rolled under the table") — it describes something underneath, not a place of origin within a region.

• "at" expresses a specific, exact point treated as a single spot (e.g., "waiting at the gate", "arrived at the station") — it does not fit an extended area or mountain range like "the Himalayas".