Husserl's fifth Cartesian Meditation concerns the phenomenological reduction, where the subject (ego) reduces the world to its own transcendental subjectivity, seeking to understand the constitution of experience. However, this reduction does not fully isolate the subject from the other; instead, it reveals how the subject relates to the world and others.
Option (A) is correct because the reduction does not sever the subject from the other but rather places it in a position where it can understand its relation to the world.
Option (B) is incorrect because the reduction does not cut off the subject from the other in a fateful or absolute sense. It rather reveals the ways in which the other is constitutionally related to the subject.
Option (C) is incorrect because Husserl does not claim that the reduction can never be fully achieved, but that it is a methodological step in the search for the essential structures of experience.
Option (D) is incorrect because understanding the constitution of the objective world is exactly what the reduction attempts to achieve by focusing on the subject's experience.
\[
\boxed{\text{The correct answer is (A).}}
\]