What metric indicates the number of households watching a program at a given time?

The correct answer is that the metric indicating the number of households watching a program at a given time is ratings. Ratings provide a measurement of the total audience size of a television program, expressed as a percentage of the total possible viewers, which includes all households with a television. This metric is essential as it helps advertisers understand the program's reach and viewership, influencing advertising rates and overall network programming strategies.

Shares, while related, measure the percentage of households that are watching a program compared to all households that are currently watching television at that moment. Demographics pertain to the statistical characteristics of the viewers (such as age, gender, and income), which help in understanding the audience's makeup but do not directly indicate the number of households watching. Concepts generally refer to ideas or plans and are not a metric in this context. Thus, ratings serve as the primary metric for assessing audience size.