Which species is a Lewis acid?
A Lewis acid accepts an electron pair. Fe³+ can accept electron pairs due to its positive charge and incomplete octet. This follows from NCERT principle where the relation explains the outcome clearly for students in simple steps.
Ref: NCERT Chemistry Textbook for Class XI and XII, Chapter: Some Basic Concepts, Structure of Atom and Periodicity, Topic: Mole concept, atomic models and periodic trends.