default or selected kernelversion does not have config value CONFIG_HISAX_EURO.
Result is shown for kernelversion 6.9.9

HiSax Support for EURO/DSS1

configname: CONFIG_HISAX_EURO

Linux Kernel Configuration
└─>Device Drivers
└─>ISDN support
└─>Old ISDN4Linux (deprecated)
└─>Passive cards
└─>HiSax Support for EURO/DSS1
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
Say Y or N according to the D-channel protocol which your local
telephone service company provides.

The call control protocol E-DSS1 is used in most European countries.
If unsure, say Y.