💡 Detailed Explanation
"～ものだ" is used to express common sense, something that is natural or expected. This sentence states a general truth that "it is natural for parents to worry about their children," so "ものだ" is the most appropriate choice. "わけだ" indicates a logical reason or conclusion. "ことだ" is used for giving advice or commands. "はずだ" expresses a strong expectation or assumption. These do not fit the context.