The human embryonic heart begins to beat at around the same time that it`s formed which is in the fourth week of pregnancy, hence option 3 is correct.

Option 1 and 2 are incorrect as this is too early in the developmental process. During the first and third weeks, the embryo is just beginning to form and the structures that will turn into the heart have not developed yet.

Option 4 suggests that the heart starts beating in the sixth week of development, but this is also incorrect. By the sixth week, the heart is not just beating, but also undergoing further significant structural changes.

Thus, the human embryo`s heart starts beating in the fourth week of its development, making option 3 the correct answer.