ANSWER:

In case you arrive before isha time to your home, then you must pray at home, as the time is not over yet. you cannot pray fard salah while you are sitting unless the time is going to expire and you can’t stop the bus. 

I have to take bus to and fro my home and office.If i do overtime (which is sometimes required) the maghrib prayer falls during that journey.I arrived at home about 20-25 minutes after the azan of maghrib.The surrounding atmosphere is not completely dark while i arrived home.So,is it permissible for me to pray after returning home or should i pray in bus while sitting?