Answer:
a. Subpoena duces tectum
Step-by-step explanation:
Options C (an informant steal the documents) and D (discrete copy of the documents) would be considered illegal and/or inadmissible in most places, so you should avoid these.
Options A (subpoena) and B (search warrant) are usually issued by a judge after considerations, so they are both legal ways to obtain something.
In a subpoena duces tecum, the court orders an individual (or a moral person) to produce (usually very specific) documents to court or face penalty.
A search warrant is executed by law enforcement officials who are seeking information to help in their investigation, usually not highly specific (not 'search for a copy of the April 10 minutes of the meeting.').