CONFIG_RISCV_M_MODE is not available for the selected architecture x86.
Result is shown for architecture riscv

CONFIG_RISCV_M_MODE (not configurable)

configname: CONFIG_RISCV_M_MODE

Linux Kernel Configuration
└─>RISCV_M_MODE
In linux kernel since version 4.14.326 (release Date: 2023-09-23)