One argument for opposing antidiscrimination laws regarding sexual orientation is the claim that the amendment of the u. s. constitution forbids all discrimination against any group, including, by default, people of any sexual orientation.

The answer is  Fourteenth Amendment.
The fourteenth amendment of United States guarantees equal protection of the laws. Which means no citizens shall have their personal rights/liberties threaten simply because they do not belong in majority religion, race, ethnicities, or gender.

14th Amendment to the U.S. Constitution. It granted citizenship to “all persons born or naturalized in the United States,” which included former slaves recently freed. In addition, it forbids states from denying any person "life, liberty or property, without due process of law" or to "deny to any person within its jurisdiction the equal protection of the laws.

Idont know which one is right   but i know that c is wrong and a might be wrong