A child incorrectly calculates 785 - 143 = 544. What is the most valid assessment of her error?

The assessment of the child's error as a lack of mastery of the basic subtraction facts is appropriate because the calculation involves a fundamental arithmetic operation that should be within the grasp of a child at this level. The child obtained an incorrect result, which suggests that she may not have a solid understanding or recall of the subtraction process necessary to solve the problem accurately.

Basic subtraction requires familiarity with borrowing and subtracting across zero, especially with numbers like 785 and 143, where it is necessary to manage different place values effectively. An incorrect answer indicates she may struggle with the foundational concepts that underpin these computations.

While other options might highlight the possibility of confusion or minor mistakes, they do not adequately address the underlying issue that the child might not have fully mastered the essential subtraction techniques necessary for solving the problem at hand. Mastery of basic subtraction facts is critical for successfully performing operations involving larger numbers, ultimately making option C the most valid assessment.