How to implement 8x1 multiplexer using 3x8 decoder and 3-state buffer
a) You are wrong. Your truth table has several wrong entries, e.g. 0001 should be 0, not 1.

b) The drawn in OR gate though is wrong. With all but one input floating it is anyones guess what the output of an OR gate would be. Simply connect all the outputs together without further logic.

c) Your idea of connecting 1 and NOT D input signals to the 3-state buffers though is right. Except your bad truth table makes the result wrong. You also need 0 input for some.