The word COMPATIBLE is divided into two parts of equal lengths : COMPA & TIBLE I : COMPA - BQNPD The diagonally opposite element is substituted by its immediate following alphabet in the English alphabetical series. => C is substituted by D at its diagonally opposite position. Similarly, O is substituted by P at its diagonally opposite position and similarly for M, P and A. II : TIBLE - DKAHS The diagonally opposite element is substituted by its immediate preceding alphabet in the English alphabetical series. => T is substituted by S at its diagonally opposite position. Similarly, I is substituted by H at its diagonally opposite position and similarly for B, L and E. Following the above method, code for STABILISED is : STABI - JCBUT LISED - CDRHK => STABILISED - JCBUTCDRHK