This is the 3477th most frequent Russian word.
профессию
"Profession."
The word 'профессию' here is used in the accusative case as it follows the verb 'выбрал' (chose), indicating the direct object of the choice.
Он выбрал профессию врача.
He chose the profession of a doctor.
In this sentence, 'профессию' is again in the accusative case, as it follows the verb 'изучать' (to study), showing what is being studied.
Изучать профессию инженера требует упорства.
Studying the profession of an engineer requires persistence.
Here, 'профессию' is in the accusative case because it is the direct object of the verb 'уважать' (to respect).