Question

why IST has been chosen

Answer 1

Because it is favourable all over the world and it is divisible by 7.15 that is taken by many countries as standard time.

Answer 2

$\huge\mathfrak\red{ANSWER }$

IST stands for Indian Standard Time.

82 degree 30 minutes  is selected as standard meridian because it lies almost in center of the india. it has prevented alot of confusion from place to another and has brought harmony in timethroughout the country.the local time ateach longitutde is different so, in order to follow one timing we have accepted 82degree 30minutes as standard meridian.