Can I use a 15" alloy with 205/50r15 tyre in maruti suzuki ritz where the actual rim size of the car is 14"?
Short answer; maybe. It might work if certain details are met, of which you did not provide.
First, the bolt pattern and center hole must be correct for the rim to mount safely (5.5Jx14 ET45).
Second, the mounted tire must fit in the wheel wells without interference. The 205/50R15 is close to the size of the stock tires listed on the internet that I could find (165/80R14 or 185/70R14 on 2009 to 2019 models).
YOU will have to determine if they will fit properly and the steering swing clears all suspension and body components.