Final answer:
Along a a. fault, movement is mostly vertical, which can result in a lengthening of the crust. Anticlines and synclines are types of folds resulting from compressional forces and do not typically lead to the lengthening of the crust. Rifting areas are associated with the extension and thinning of the crust.
Step-by-step explanation:
The correct answer to the question “Along a _____, movement is mostly vertical, resulting in a lengthening of the crust involved” is a) Fault. This is because along faults, the movement of the Earth's crust can be vertical, and when this occurs with normal faults during extension, the crust is lengthened.
Anticlines and synclines refer to types of folds in the Earth's crust, which are created by compressional forces and result in the bending of rock layers. Anticlines are folds where the oldest rock layers are at the center and the rocks get progressively younger towards the outside of the fold.
Synclines are the opposite, with the youngest layers in the center and the oldest on the outside. These forms of deformation do not typically result in the lengthening of the crust but rather in its shortening or folding. A rifting area, on the other hand, is associated with extension and thinning of the crust, not vertical movement.
Therefore, to answer the student's question, faults are the geological structures along which movement is mostly vertical and can result in a lengthening of the crust.