What is considered a perfect vacuum?

A perfect vacuum is defined as a space that is devoid of matter, meaning it has zero pressure. In practical terms, pressure is often measured in inches of mercury (inHg) when referring to vacuum conditions. The standard atmospheric pressure at sea level is approximately 29.92 inHg.

In the context of the options provided, -29.9 inHg is indeed the closest representation of a perfect vacuum when considering standard atmospheric pressure.

When we talk about vacuum measurements, values are often expressed as negative vacuum pressure beneath atmospheric pressure. Therefore, -29.9 inHg indicates a very high vacuum, approaching the concept of a perfect vacuum.

Conversely, other values such as -30 inHg and -15 inHg do not meet the standard definition for a perfect vacuum, as they suggest either a pressure that is less than -29.92 inHg, which exceeds the limits of practical vacuum technology, or a significantly higher pressure when compared to -29.9 inHg. Thus, option -29.9 inHg is considered the most accurate measurement in this scenario.