How to Design a combinational circuit using a suitable decoder and OR gate
The decoder is a given.  It is takes 4 inputs, and has 16 outputs. Outputs could  be labeled 0 to 15. For any combination of inputs exactly one of the outputs will go high. Draw a block with 4 inputs on the left and 16 outputs on the right. That is given in the setup to the problem. For example, a 74LS154 IC is a decoder (but the outputs are active low).

The exercise is about combining the decoder and OR gates to get the output shown in the question.