Given that this is in an instructional context, I would use "elective" here.

For example, see:

The PADI Advanced Diver Course consists of five dives, two required dives in Under Water Navigation and Deep Diving, and three elective dives. The cost for the class is $355. The student may choose the elective dives from the following list...

http://www.divestjohn.com/scuba.advanced.html

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Note added at 7 mins (2009-07-02 18:29:46 GMT)
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ADVANCED DIVER CERTIFICATION: The core curriculum of this course is a Deep Dive and a Navigation Dive. From there, divers will be able to choose three elective dives in order to complete the five dives required for this certification. Your textbook is used to introduce you to each of the dives you will complete and works as a briefing for each dive.

http://www.majesticholidays.com/-dest-PARA-vendor-ADVSC_-_pa...

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Note added at 7 mins (2009-07-02 18:30:30 GMT)
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Ready for some new adventures? The SSI Advanced Open Water Program is a 2 day “hands on” course in which you learn while you dive. A variety of five, ocean dives and brief academic sessions will allow you to explore deep diving, night diving, and underwater navigation as well as two specialty areas of particular interest to you. With your instructor you will select the 2 elective dives from specialties such as search and recovery, u/w naturalist, multilevel diver, peak performance buoyancy, u/w photography, diver propulsion vehicle, and boat diving...

http://www.mauidiveshop.com/Store/detail.aspx?ID=53

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Note added at 9 mins (2009-07-02 18:31:58 GMT)
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Next, complete all the training modules in the workbook to prepare for the required dives. The advanced open water certification requires a navigation dive, a deep dive, a night dive and two elective dives. The navigation dive will consist of using both a compass and natural references to navigate underwater. A buddy pair will be asked to navigate a square, rectangle, or triangle. When performing the deep dive, the most important things to remember are planning for the short no-decompression limits and rapid use of air supply. The night dive will be dependent on successfully performing descents, communication skills, and ascents. The two elective dives can be chosen from altitude, boat, drift, dry suit, multilevel, search and recovery, underwater naturalist, underwater photography, or wreck diving.

http://www.kennesaw.edu/clubs/scuba/DivingTips1.htm

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Note added at 9 mins (2009-07-02 18:32:32 GMT)
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Note: Based on my research, it appears that "elective dives" is the standard industry term in English.