Final answer:
Option a. 1.5 represents the largest pitch and would stretch the x-ray helix the most by resulting in the longest distance between two turns of the helix.
Step-by-step explanation:
The question 'Which of the following values for pitch would stretch the x-ray helix the most?' refers to the geometry of a helical structure, possibly in the context of x-ray diffraction or related concepts in physics. Generally, the 'pitch' of a helix refers to the height of one complete helix turn, and increasing the pitch would stretch the helix longitudinally. Therefore, among the given options (1.5, 1.0, 0.8, 0.5), the highest value represents the largest pitch. Option a. 1.5 would stretch the x-ray helix the most since it represents the longest distance between two turns of the helix, resulting in a 'stretched' appearance.