When impulses reflected by the soil are counted by a gauge on a machine, this is known as a _______ test?

The correct choice relates to a specific type of test that utilizes electromagnetic radiation to assess soil properties. In the context of soil testing, the nuclear density test employs radioactive isotopes to emit gamma rays into the soil. These rays are then reflected back to a gauge, allowing for the measurement of soil density, moisture content, and other important characteristics.

This method is effective because it enables non-destructive evaluation and provides an instant reading, which is particularly useful in the field for making informed decisions about soil quality and suitability for various types of construction and compaction work. The nuclear density test's ability to provide real-time data makes it an advantageous choice for engineers and contractors.

In contrast, other methods mentioned, such as soil density tests, ultrasonic tests, and vibrational tests, focus on different principles or mechanisms that do not involve the same type of radioactive measurement and reflection.