Scenario: Kenneth and Catherine Lohmann of the University of North Carolina explored the orientation or direction-finding abilities of loggerhead sea turtle hatchlings under varying magnetic conditions. Hatchlings were placed in a tank of water in a laboratory either in the natural magnetic field of the earth OR in a reversed magnetic field in which north and south were flipped. The researchers then measured the direction that the hatchlings swam. The experiments were carried out in the dark and at night since that is when most newly hatched turtles migrate from their sandy nests into the ocean. What is the variable being tested in this experiment

Variable being tested: The direction taken by the hatchlings under different magnetic fields.

Explanation:

The independent and the dependent variables are the two principal factors needed in an experiment.  

In the exposed example, we know that

Goal: To explore the orientation or direction-finding abilities of loggerhead sea turtle hatchlings under varying magnetic conditions.

Experiment:

The hatchlings were placed in water tanks in the laboratory, under dark conditions and during the night.

Observations were done about the swimming direction of the hatchlings

Variable being tested: The direction taken by the hatchlings under different magnetic fields.

In this experiment, the independent variable is the magnetic field conditions. The alternative conditions -how this variable is modified- are the Natural magnetic field and Reversed magnetic field. Researchers are no interested in the magnetic field, they are interested in analyzing how the change in the magnetic field affects the turtles´ orientation. Then, we can assume that the dependent variable is the direction taken by the turtles. It depends on the change in the magnetic field. Researchers are interested in testing how the change in the magnetic field affects the orientation of the turtles. This is the variable being tested in the experiment.

the answer to your question is:

c. complementary base-pairing