Is decoder 2-4 a complete system?
In order for a gate/module/system to be Functionally Complete, it must be possible to make any logic gate from a combination of this gate/module/system.

NAND is a standard example of functional completeness as it is possible to make all gates from a network of NAND gates. The same is true for NOR gates.

To prove functional completeness of a system, you either need to show a way in which every other gate can be made from this system, or show that you can make a NAND gate or a NOR gate from your system. The former proves it the long winded way, the latter proves it by relying on the knowledge that NAND/NOR are complete.

In your case, for a 2-4 Decoder, it is possible to make a NOR gate. The output \$D_0\$ is the NOR of the inputs \$A\$ and \$B\$. Therefore a 2-4 decoder is functionally complete.