The MUX and its observation register

One output, two choices

A multiplexer chooses one input and routes it to an output. Our two inputs are a and b; sel decides which one is selected.

In assign y = sel ? b : a;, sel=0 selects a and sel=1 selects b. The combinational output follows its input without storing a value at a clock edge.

sampled_y is a register. It captures y at a rising clock edge and holds that value until the next rising edge. Compare these two behaviors in the waveform.

With a=0 and b=1, when do y and sampled_y change as sel goes from 0 to 1?