Which DNA crystal feature told Crick the two strands were antiparallel?

The DNA crystal feature that told Crick the two strands were antiparallel was the space group.

In X-ray crystallography, a crystal's space group describes the symmetry operations compatible with the repeating arrangement of molecules. Rosalind Franklin's analysis of DNA crystals identified the relevant space group, giving Francis Crick an important structural constraint. The symmetry was consistent with two strands running in opposite chemical directions rather than parallel ones.

“Antiparallel” refers to the orientation of DNA's sugar-phosphate backbones: one strand runs 5′ to 3′, while the other runs 3′ to 5′. This arrangement is separate from the helix's pitch, which describes how far the structure advances during a turn. Base ratios helped suggest complementary pairing, and water content supplied information about hydration, but neither was the crystallographic clue named in this question.

The space-group evidence was part of a larger chain of reasoning. Watson and Crick combined crystallographic constraints with diffraction images, chemical knowledge, Chargaff's base relationships and model building. Together, these clues produced the double-helix model published in Nature in 1953.