Explanation

Creating a filter: in this question, the underlined phrase appears to be defect-free. The phrase starting with "which" refers to "part," as an additional description of the part of the circuit that has been defined by "that detects a positive signal." Choice (A) may be correct, so we'll look for defects in the other choices and come back to it.

Finding objective defects: choice (B) incorrectly uses the grammatically plural verb "take" to refer to the grammatically singular "part." Choice (C) distorts the meaning: it indicates, nonsensically, that this special part of the circuit achieves its detection of the positive signal by diminishing it. Choice (C) is out. In (D), using the infinite with the verb "to be" in this way conveys a semi-moral sense of obligation that is appropriate only for humans and other moral actors. So (D) is out. And speaking of people, (E) uses the relative pronoun "who" as if the circuit were a person. The correct answer is (A).