The correct answer is A. Balusters.
Balusters are the vertical members fixed between the treads and handrail of a staircase. They are typically made of wood, metal, or stone, and are often decoratively carved or turned. Balusters serve to support the handrail and to prevent people from falling off the stairs.
Strings are the horizontal members that support the treads and risers of a staircase. They are typically made of wood or metal, and are often hidden from view by the treads and risers.
Newel posts are the vertical members that support the ends of the strings and the handrail. They are typically made of wood or stone, and are often decoratively carved or turned.
Soffits are the undersides of stair treads and landings. They are typically made of wood or drywall, and are often painted or finished to match the rest of the staircase.