CONFIG_RTS7751R2D_PLUS is not available for thedefaultarchitecture x86.
Result is shown for architecture sh


configname: CONFIG_RTS7751R2D_PLUS

Linux Kernel Configuration
└─>RTS7751R2D Board Revision
In linux kernel since version 2.6.24 (release Date: 2008-01-24)  
Selecting this option will configure the kernel for R2D-PLUS.

R2D-PLUS is the smaller of the two R2D board versions, equipped
with a single PCI slot.