Apple Watch Fitness app

Aerobics is a type of exercise that strengthens your heart and lungs in addition to the muscle groups you're using during your workout. Running, cardio kickboxing, cycling, swimming, jump rope, and dance are all types of aerobic exercise. Other types of aerobic exercises include:

• Swimming.
• Cycling.
• Using an elliptical trainer.
• Walking.
• Rowing.
• Using an upper body ergometer (a piece of equipment that provides a cardiovascular workout that targets the upper body only).

Choose the workout type that you mostly use or switch between them.

If you mean that you're taking what's often called an "aerobics class", I'd suggest "Dance" as the workout type.

Glad it helped!