What Does the Term "Conservation" Refer To,regarding a Child's Cognitive

What does the term "conservation" refer to,regarding a child's cognitive ability

A)  ability to understand that simply because something isn't visible doesn't mean it no longer exists 
B)  ability to solve moral dilemmas in the most economical manner possible 
C)  assimilating new experiences into an existing memory structure in order to conserve memory space 
D)  awareness that changing the shape of something doesn't alter its volume or mass