Correct Answer - A
According to question ,
Length of platform = `21 xx 19 - 216 = 183` meter
Let x coaches are added
`216 + 183 + 21 x = 21 xx 26`
`implies 21 x = 147`
`implies x = 7`
So , 7 coaches were added to the train .
Alternative Solution :
Let x coaches were added
`therefore` distance cover = x `xx 21`m
also , time increase = 7 sec , travelling at 21 m/s .
`therefore d = 21 xx 7 = x xx 21` m .
`implies x = 7 ` coaches