Never learned pascals triangle. So I'm not sure what the question means.
  
 By subsituting real numbers (specifically integers)
 If n = 2
 x^2 + 2x + 1 and n choose k will be 2
  
 then the variable will be x
  
 If n = 3
 (x + 1)(x^2 +2x + 1)
 x^3 + 2x^2 + x + x^2 + 2x + 1
 x^3 + 3x^2 + 3x + 1
 n choose k will be 3
  
 then the variable will be x
  
 So far I think the answer is x