Which mode is most commonly used for voice communications on the 160-, 75-, and 40-meter bands?
Why By long-standing amateur convention, SSB voice below 10 MHz uses lower sideband, and above 10 MHz uses upper sideband. The 160-, 75- and 40-meter bands are all below 10 MHz, so they are LSB bands. This is a custom rather than an FCC rule, but everyone follows it so that stations can hear each other properly.
Watch out Upper sideband is the convention for 20 meters and higher (and for VHF/UHF SSB and all HF digital work), not for the low bands. 'Suppressed sideband' and 'double sideband' are not operating modes you select on a radio; SSB by definition suppresses the other sideband and the carrier.
Below 10 MHz go Low (LSB), above 10 MHz go Up (USB). 160/75/40 are all below, so LSB.