Explanation: Monitoring a frequency to determine it's clear before transmitting is the most effective preventative measure against causing interference. FCC Part 97.101(c) mandates that amateur stations must cooperate in selecting transmitting channels and make every reasonable effort to avoid causing interference. Transmitting on an occupied frequency, regardless of power, will disrupt ongoing communications. Option A is incorrect because an omni-directional antenna radiates equally in all horizontal directions and cannot be "pointed away" to avoid interference. Its use doesn't prevent transmitting on an occupied frequency. Option B, reducing power, is good practice (using minimum necessary power per 97.313(a)), but transmitting at *any* power on an active frequency still constitutes interference, even if weak. It doesn't prevent the interference from occurring. Option D describes a reactive response *after* interference has been caused and reported. While necessary, it doesn't prevent the initial interference, which is the goal of the question.