Correct Answer - C
Let T be the final temp.of each gas in equilibrium
Heat gained by di-atomic gas = Heat lost by monoatomic gas
`n_(1)C_(upsilon_1) (T-400)=n_(2).C_(upsilon_2) (700-T)`
`2 xx 5/2 R (T-400) = 2 xx 3/2 R (700-T)`
or, `5T - 2000 = 2100-3T`
`8T = 4100`
`T = 4100/8 = 512.5K`.