Cipher text for ISRO will be RJCH i.e. Option a), which I calculated as follow : A maps to 0,B maps to 1,....., I maps to 8,..., Z maps to 25. Now given k = 7, and c(m) =( k.p(m)+13) mod26 c(I) =( 7x8+13)mod26 c(I) =(56+13)mod26 c(I) =69mod26=17 Now since 17 corresponds to R so c(I) = R. Similarly you can do for S,R and O.