___________ is one of the earliest forms of a self-adjustment strategy used in splay trees, skew heaps.

___________ is one of the earliest forms of a self-adjustment strategy used in splay trees, skew heaps.
 
(a) Union by rank
 
(b) Equivalence function
 
(c) Dynamic function
 
(d) Path compression
 
Origin of the question is Trees topic in division Trees of Data Structures & Algorithms I
 
I had been asked this question at a job interview.