Final answer:
In gestational trophoblastic disease, the commonly observed abnormal growth is a hydatidiform mole, which is an abnormal growth of placental tissue characterized by the proliferation of trophoblasts.
Step-by-step explanation:
In gestational trophoblastic disease, the commonly observed abnormal growth is hydatidiform mole. A hydatidiform mole is an abnormal growth of placental tissue in the uterus that forms a mass of cysts. This abnormal growth is characterized by the proliferation of trophoblasts, which are cells responsible for forming the placenta. The hydatidiform mole can result from an abnormal fertilization event, leading to the development of an abnormal pregnancy.