Final answer:
The correct transaction code for searching the NICS Denied Transaction File is DQ. This code is specifically for 'Denied Query', which is used to look up denied transactions in the NICS system.
Step-by-step explanation:
To search the NICS Denied Transaction File, the correct transaction code that should be used is d) DQ. The National Instant Criminal Background Check System, commonly known as NICS, is a system for determining eligibility to purchase firearms in the United States. Different transaction codes are used for specific types of queries within the system. The code 'DQ' stands for 'Denied Query', which is the specific inquiry for searching denied transactions in the database. In this context, other options such as TQ, IQ, and NQ refer to different types of queries and would not be appropriate for searching the Denied Transaction File within NICS.