Final answer:
Sequential microfilm is insufficient for patients with multiple admissions because it organizes records by the order of creation, making it difficult to access all records for a patient scattered across multiple microfilms.
Step-by-step explanation:
When patients have multiple admissions on microfilm, the format that is insufficient is a) Sequential microfilm. Sequential microfilm organizes records in the order they were received or created, which can be problematic when trying to locate all records pertaining to a patient with multiple admissions, as their records could be scattered across the microfilm. On the other hand, b) Unit record microfilm is organized by patient and would include all admissions for a patient in one place. c) Serial microfilm and d) Patient-oriented microfilm are not standardly recognized types in microfilm categorization and thereby do not have defined characteristics to consider in this context.