Final answer:
The sidechain compressor should be placed on the Bass synth track to 'duck' the sound whenever the kick drum hits, allowing the kick to stand out in the mix.
Step-by-step explanation:
When using a sidechain compressor to make a bass synth sound 'duck' whenever a kick hits, the compressor should be placed on the Bass synth track (A). The sidechain input of the compressor is triggered by the kick drum signal, reducing the volume of the bass synth whenever the kick drum hits. This technique is used to allow the kick drum to stand out more in the mix by momentarily lowering the bass level, which is commonly used in electronic and dance music to achieve a pulsing effect.