In a leftist heap, the null path length of a null node is defined as?

In a leftist heap, the null path length of a null node is defined as?
 
(a) 0
 
(b) 1
 
(c) null
 
(d) -1
 
I’m obligated to ask this question of Heap topic in chapter Heap of Data Structures & Algorithms I
 
This question was posed to me in an interview for internship.