Measuring public opinion becomes highly expensive when

Measuring public opinion becomes highly expensive when a "public opinion poll is conducted". Public opinion polls involve surveying a representative sample of the population to gather information about their views on a particular issue or topic. The cost of conducting a poll can vary depending on factors such as the size of the sample, the method of data collection, and the number of questions asked. In order to obtain a reliable and accurate result, public opinion polls must be conducted with high-quality methods and by trained professionals. These measures can increase the cost of conducting a poll. Therefore, the correct option to select in this scenario would be "public opinion poll is conducted".