CONFIG_RISCV_ISA_C is not available for thedefaultarchitecture x86.
Result is shown for architecture riscv

Emit compressed instructions when building Linux

configname: CONFIG_RISCV_ISA_C

Linux Kernel Configuration
└─>Platform type
└─>Emit compressed instructions when building Linux
In linux kernel since version 3.10 (release Date: 2013-06-30)  
Adds "C" to the ISA subsets that the toolchain is allowed to emit
when building Linux, which results in compressed instructions in the
Linux binary. This option produces a kernel that will not run on
systems that do not support compressed instructions.

If you don't know what to do here, say Y.