The robotics team should submit a patent (A) to protect their invention if the functional arm is a new and useful invention or process. Patents protect inventions and grant the inventor exclusive rights to use, sell, and license the invention for a certain period of time.
Trademarks (B) protect brand names and logos, copyrights (C) protect original works of authorship, and trade secrets (D) protect confidential business information. None of these options would be appropriate for protecting a new invention like a functional arm for a robot.