The correct answer is A. swallowed.
The other options are incorrect because they do not accurately describe the situation. Option B, just swallowed, implies that the baby swallowed the safety pin very recently, which is not necessarily the case. Option C, had just swallowed, implies that the baby swallowed the safety pin some time ago, which is also not necessarily the case. Option D, was just swallowing, implies that the baby was in the process of swallowing the safety pin, which is not the case since the woman said that the baby had already swallowed it.
The correct answer, A, swallowed, is the most accurate description of the situation. It is clear from the woman’s statement that the baby had already swallowed the safety pin, and there is no reason to believe that she is mistaken.