Ra,Rb,Rc as a group and Rd,Re,Rf as another
one should be between the reference 3.3V and ground, the other should be from the sensor's output.
Also if you use an op-amp for comparator, it may be ok however the buzzer will not sound, but if you will use a comparator like LM339, the LED's should be between +V and comparator out pin with a series resistor to limit the current.
The sensor sends IR and reads its reflection from surface. The data sheet says it used triangle calculation to avoid reflection differences between shiny and dull objects.