Keys are based on contrasting characters generally in pairs called

The correct answer is couplet.

In biological taxonomy, a key is a structured tool used to identify organisms based on their observable characteristics. A dichotomous key is the most common type.

A dichotomous key consists of a sequence of choices. Each choice is presented as a pair of contrasting, mutually exclusive statements or characters. This pair of contrasting statements is called a couplet.
Within a couplet, each individual statement or choice is referred to as a lead. By selecting the lead that best describes the organism, the user is guided to either another couplet or the final identification of the organism.

Let's look at the other options to understand why they are incorrect:
- dimer: A term used in chemistry and biochemistry to describe a molecule or molecular complex consisting of two identical or similar subunits (monomers) linked together.
- duplex: A general term meaning double or twofold (often used in genetics to describe DNA structure or in telecommunications).