I prefer:

char hex_digit_to_ascii( unsigned char hex_digit )
{
   const *char ascii = "0123456789ABCDEF" 

    
   if( hex_digit < 16 )
   {
      return ascii[hex_digit];  
   }
   else
   {
      // It is not a valid value for a single Hex digit!
      // You decide what action to take here!
   }
}

I like math solutions to patterns. But it is not always the better solotion