Combining Lead pipe with Zinc or Copper pipes in a domestic water installation
The focus of the analysis should be first on the anodic index (see, for example, table here), which relates to the electrochemical corrosion potential of the metal serving as the anode. Second, materially important is the current state of the old municipality Lead pipes.
Per the source, the anodic index of Pb is high (see chart) above copper, so historically it has already electrochemically corroded likely inducing a protective Lead oxide coating in the presence of dissolved oxygen.
However, Zn metal is more anodic than the Pb anode, and as such, would readily corrode and may introduce high levels of Zn ions (even levels considered problematic/toxic).
As such, copper metal, which is lower in the anodic index relative to the Pb electrode, is likely a better choice to preserve the current state of affairs, which is in apparent agreement with the reputed cited textbook 'correct' answer.