Which of the following represents a probable sign of pregnancy?

A positive pregnancy test is considered a probable sign of pregnancy because it provides a biological indication of the presence of hCG (human chorionic gonadotropin) in the body, which is produced after a fertilized egg implants in the uterus. This hormone can be detected in urine and blood tests, and a positive result typically confirms that pregnancy has occurred.

On the other hand, morning sickness, excessive fatigue, and frequent urination are common symptoms associated with pregnancy but are not definitive indicators on their own. These symptoms can be caused by various factors unrelated to pregnancy, such as stress, dietary changes, or underlying medical conditions. Therefore, while they may suggest the possibility of pregnancy, a positive pregnancy test unequivocally confirms it.