I need to make a quad for taking a payload of 2.5 kg with a flight time of 15mins.... experts please guide me....

May I ask what's are you planning to carry? If you are carrying anything expensive like a DSLR camera go for a hexacopter. On quad if one motor fails the quad would fall down like a rock. On hexa you would still have a chance to land it safely.

sir just gonna attach an arm to it... and carry a small parcels.... this is for my friends project

Okay.
You can go with this configuration
Frame: F450
Motors: Sunnysky X2216 880kv(Go with a lower KV and larger prop)
ESC: 30A Hobbywing
Props: 11-12"
Battery: 3S 4000mAh~5000mAh
Should give you your desired flight time(may be +-2min)

With that payload I guess you should be looking for a motor with 650kv~750kv for desired flight time

Agreed with the above statement.(Himadri Roy's statement)
Large diameter pitch props at lower KV ratings And 4000mah+ will easily give you around 15 minutes of flying time and decent thrust to carry this much payload....

With 11" prop it can produce thrust up to 1100gm or 1200gm.....That means 4 motors would be easily be able to carry around 4.5kg to 5kg(Which is somewhat meeting this condition/thumb rule for calculating total thrust needed , Assuming you wont be performing maneuvers ) This will give you stable flight.

But keep this in mind, Do not over use ur lipo battery.Use 80% of total charge.

In another words the Voltage ratings for each cell should be atleast 3.3v(and it shouldnt go down than this).......And u can use https://www.sparkpcb.com/robotics/quadcopters/lipo-battery-tester-and-low-voltage-alarm.html to keep voltage in check....Or get a good Transmitter which can show you live status of battery.......