The solution of $X$ having excess of caustic potash is used to detect ammonia. Which of the following is $X$?

Question

$$\begin {array} {1 1} (a)\;K_2[HgI_4] & \quad (b)\;[Co(NH_3)(_6]Cl_3 \\ (c)\;K_3[F_e(NCS)_6] & \quad (d)\;[Co(NH_3)_5SO_4]Br \end {array}$$