The correct answer is A. Nail matrix.

Beau’s lines are transverse depressions in the nail plate that result from a temporary arrest in nail matrix activity.

The other parts of the nail are not responsible for the formation of Beau’s lines. The nail plate is the hard part of the nail, and defects in the nail plate are associated with true leukonychia. The nail bed is the skin beneath the nail plate, and defects in the nail bed lead to apparent leukonychia. The proximal nail fold is the skin covering the base of the nail, and trauma to the nail fold can lead to paronychia. The hyponychium is the skin under the free edge of the nail, and ventral pterygium affects the hyponychium.

References:

Bolognia JL, Schaffer JV, Duncan KO, Ko CJ. Dermatology Essentials. 2nd ed. Elsevier; 2022.
Lee YJ, Yun SK. Unilateral Beau’s lines associated with a fingertip crushing injury. J Dermatol. 2014;41(7):914-916. doi:10.1111/j.1346-8138.2005.tb00871.

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