A 1RK is one room, a kitchen and a bathroom. A 1BHK adds a separate hall, so it has a bedroom, a hall (living room), a kitchen and a bathroom. A 1BHK costs more and suits couples or small families; a 1RK suits one person.
Side by side
Factor
1RK
1BHK
Rooms
One room
One bedroom and one hall
Kitchen
Separate
Separate
Separate living space
No
Yes
Price and rent
Lower
Higher
Suits
One person
Couples, small families
Resale demand
Narrower
Wider
When 1RK matters
A 1RK suits a single person who wants their own place near work at the lowest cost.
When 1BHK matters
A 1BHK suits anyone who needs a separate room for guests, work or a partner.
In more detail
Compare on carpet area: some 1BHKs are barely larger than a generous 1RK.
For rental investment, check local demand for each in the locality; the site's rate pages show asking rents by locality.