Answer:
c.They rank-order the units of analysis in terms of specific variables
Step-by-step explanation:
An index is a way  of compiling one score from a variety of  questions or statement that represents a belief, feeling or attitude.
Scales on the other hand measure levels of intensity at a variable level like how much a person agrees or disagrees with a particular statement.
The one thing common to both  indexes and scales is that
They rank-order the units of analysis in terms of specific variables.