The best estimate of a child's physical maturity is

The development of the child's bones.

Explanation:

The measure of development of ones bones is called skeletal age. It is widely used in the industry to not only check the maturity of an individual but also to predict one's height and to check if there are any irregularities in one's growth. This is done via the use of x-rays of various bones to determine ones skeletal age. Also known as bone age.

"harry t. moore (1905-1951) – civil rights activist, leader, and the first martyr of the civil rights movement. fannie lou hamer (1917–1977) – leader in the american civil rights movement; co-founder of the national women's political caucus and freedom democratic party. james farmer (1920–1999) – congress of racial equality."

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