What vitamin D level indicates a deficiency?

Vitamin D deficiency is commonly defined as a serum 25-hydroxyvitamin D level below 30 ng/mL. This level indicates a lack of sufficient vitamin D in the body, which can lead to various health issues such as weakened bones, muscle pain, and increased risk of fractures. Therefore, option B (< 30 ng/mL) is the correct answer.

Options A, C, and D are not indicative of a deficiency as they are above the commonly accepted threshold for defining a vitamin D deficiency.