The correct option is B Low
Let's examine the sentence: "She is a lady of _____ birth." The sentence requires an adjective that can add to the meaning of 'birth.' Two options are given for the blank: Low and Lowly. The adjective 'lowly' means simple, plain, humble or unpretentious. The adjective 'low' refers to position with respect to social status. As the sentence talks about 'lady,' and not 'woman,' it is indicating to the social status of the person. Hence, 'low' would be more appropriate to use here with 'birth.' Here, 'low birth' would indicate lesser social status of the 'lady.' So, Option B is the correct answer.