If the major dimensions of a discontinuity in a 6-inch thick aluminum plate lie parallel to the entry surface at a depth of 3 inches, it will be best detected by:

A straight beam test is the best method for detecting a discontinuity oriented parallel to the entry surface at a specified depth. In this case, since the major dimensions of the discontinuity are parallel to the surface and located at a depth of 3 inches, a straight beam test will effectively transmit ultrasonic waves vertically into the material, allowing the waves to reflect off the discontinuity back to the transducer.

This test method provides a direct approach to identifying flaws since the sound waves travel in a straight line. The nature of straight beam testing allows for straightforward interpretation of reflected signals, which will clearly indicate the presence of a discontinuity directly beneath the surface where the transducer is placed.

In contrast, other methods such as an angle beam test are more suited for detecting flaws that are oriented at angles to the surface, which may not be as effective in this situation. A surface wave test primarily focuses on surface-breaking defects, while a time-of-flight diffraction test is more complex and often used for small, intricate defects rather than broader and more straightforward discontinuities.