MUX Notation A vs B vs Cin vs S notation
4) As says in the diagram, it is a 3-input, 1-output Boolean function implemented by using a 4-to-1 mux. The 3 inputs are A, B and C.

The bottom connection represents a 2-bit/2-line bus, with A connected to S1, B connected to S0.

C is connected to the "10"-selected-input directly and to the "00"-selected-input through an inverter.

The truth table of the Boolean function partially filled out:

A B C  Z  Comment  
0 0 0  1  Z=00-input=C' as selected by (A,B)=(S1,S0)=(0,0)  
0 0 1  0  same as above  
0 1 0  1  Z=01-input=1 as selected by (A,B)=(S1,S0)=(0,1)  
...