Final answer:
The 3D shape of DNA was discovered by James Watson and Francis Crick, who based their model on the work of Chargaff and others. Chargaff's rules, which state that adenine pairs with thymine and guanine pairs with cytosine, were integral to their discovery.
Step-by-step explanation:
The 3D shape of DNA, known as the double helix structure, was discovered by James Watson and Francis Crick. They were guided by the X-ray diffraction images produced by Rosalind Franklin among others, which ultimately contributed to their model of DNA. Consequently, the answer to the student's question is A) Watson and Crick, Adenine, Thymine, Cytosine, because while Watson and Crick elucidated the structure of DNA, it's composed of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). However, the options given are incomplete as they should also include guanine.
Based on the three chemical components of DNA: adenine, thymine, and cytosine (and guanine, which is not listed but equally important), scientists knew the following three things: that adenine pairs with thymine and cytosine pairs with guanine (Chargaff's rules); the amounts of these bases are constant within a species; and that the structure had a regular recurring pattern, which hinted at a double helix.