What indicates physiological maturity in corn?

Physiological maturity in corn is best indicated by the formation of a black layer at the base of the kernel. This black layer, which forms as the plant reaches maturity, is the result of the plant's supply of nutrients being cut off from the kernel, signaling that the kernel's development is complete and it is no longer taking in moisture or nutrients. At this point, the grain typically has reached its maximum dry weight and the physiological processes within the plant have effectively ceased.

While the turning brown of half of the leaves and grain moisture levels below 15.5% can provide context about harvesting readiness, they do not signify physiological maturity in the same definitive way as the black layer formation does. The black layer is a clear and reliable marker that indicates the corn has reached its full physiological maturity, making it the correct answer.