Final answer:
The most legal and ethical intelligence gathering technique listed is attending trade show presentations, as it is conducted in a public setting without engaging in unauthorized or unethical practices.
Step-by-step explanation:
Of the options given, the intelligence gathering technique most likely to be legal and ethical is attending trade show presentations given by a competitor's employees. This is because trade shows are public events where companies voluntarily share information about their products and services. Attending these presentations does not involve any deceptive practices, breach of privacy, or unauthorized access to information. In contrast, hiring investigators to examine a competitor's trash could involve ethical concerns such as respect for privacy; entering a production plant without authorization is clearly trespassing and, therefore, illegal; and redirecting emails is not only illegal due to unauthorized access but also violates ethical standards of fair and ethical treatment. Both in academic research and business practices, it's important to ensure the ethical treatment of human subjects and respect for intellectual property, avoiding activities like industrial espionage, fraud, or misrepresentation of facts.