Final answer:
The bits 7:4 of the A/D sample select register (AD1CHS) are used to select the positive input for sampling from A Channel (MUXA).
Step-by-step explanation:
The bits used to select the positive input for sampling from A Channel (MUXA) in the A/D sample select register, AD1CHS, are bits 7:4. These bits determine which analog input channel on the microcontroller is selected for sampling. Each bit corresponds to one of the available analog input channels, and setting a bit to 1 selects the corresponding channel for sampling.