What type of line represents a slowing production of matrix cells on the nail plate?

Beau's lines are transverse grooves or indentations that appear across the nail plate and indicate periods where there has been a temporary disruption in the growth of the nail matrix cells. These lines are significant because they can reflect underlying health issues or trauma that led to a slowdown in the production of new cells. Each Beau's line corresponds to a specific time when the matrix activity was impaired, often related to illness, stress, or nutritional deficiencies.

The other choices do not represent a slowing production of matrix cells. Dark lines may indicate pigmentation changes or other conditions but do not specifically relate to matrix cell production. Horizontal growth lines generally refer to normal variations in nail growth and may not indicate any slowing. Indentation lines could relate to other growth interruptions but are less specifically identified with the health implications that Beau's lines represent. Thus, Beau's lines serve as a clear indicator of previous disturbances in nail growth, aligning directly with the question regarding the production of matrix cells.