Explanation
Correct answer: D.
Choice D is the best answer. "Specifically" logically signals that the information about Pleasant’s 1866 lawsuit provides specific, precise details elaborating on the previous point that Pleasant successfully challenged discrimination in California.
Why the other choices are wrong
Choice A
Choice A is incorrect because "for this reason" illogically signals that Pleasant’s 1866 lawsuit was a result of her successful challenge to discrimination in California. Instead, this sentence provides specific details elaborating on her challenge to discrimination in the state.
Choice B
Choice B is incorrect because "then" illogically signals that Pleasant’s 1866 lawsuit was subsequent to or resulted from her successful challenge to discrimination in California. Instead, this sentence provides specific, precise details elaborating on how she challenged discrimination in the state.
Choice C
Choice C is incorrect because "in addition" illogically signals that the information about Pleasant’s 1866 lawsuit is merely additional to the previous point that Pleasant successfully challenged discrimination in California. Instead, this sentence provides specific details elaborating on how she did so.