Final answer:
The correct stage is the formal operational stage (A), marking the period in adolescence when individuals develop abstract thinking and can handle idealism and possibilities.
Step-by-step explanation:
The main answer to the question "During the beginning of the ________ stage, when assimilation dominates, an adolescent's thoughts are full of idealism and possibilities" is A) formal operational. This stage in Piaget's theory of cognitive development begins around the age of 11 and continues into adulthood. During the formal operational stage, adolescents develop the ability to think abstractly. This includes the capacity for higher-order thinking skills such as logical thinking, imagining hypothetical situations, and considering multiple outcomes or solutions. Unlike earlier stages, where children's thinking is tied to concrete and tangible objects and events, the formal operational stage allows for more sophisticated moral and philosophical reasoning, reflecting on concepts such as justice, freedom, and human rights.In contrast, earlier stages like the preoperational stage and the concrete operational stage feature more limited forms of thinking. The preoperational stage is characterized by the use of symbols and the initial use of language but lacks the ability to perform operations based on logic or conservation. The concrete operational stage advances children's thinking to logically understand real events, but it's still not equipped to handle abstract or hypothetical reasoning.By the time adolescents reach the formal operational stage, they are beginning to form their own identities, as Erikson outlined in his theory of psychosocial development, and their cognitive capabilities mirror this complexity and capacity for nuanced understanding.