A tumor has the following characteristics. Retrobulbar location within the muscle cone, well defined capsule, presents with slowly progressive proptosis, easily resectable, occurs most commonly in the 2nd to 4th decade. Most likely diagnosis is -

Cavernous hemangioma is the most common benign, intraconal tumor in adults. Hemangiopericytoma is also retrobulbar intraconal lesion found in adults but is a rare tumor Moreover owing to its tendency to invade the adjacent tissues, its margins are less distinct than cavernous hemangioma. Rest 2 options i.e. Dermoid and Capillary hemangiomas are primarily extraconal lesions.