Material Science

Step 1: Hume-Rothery Rules.
The Hume-Rothery rules help determine the conditions under which two metals will form a solid solution (miscibility in alloys). The key criteria are: 1. The atomic radii of the two metals should not differ by more than 12.5%. 2. The metals must have the same crystal structure. 3. The chemical properties of the metals should be similar.

Step 2: Analyze the Options.
- (A) The metallic radii of Cu and Au differ by only 12.5%, satisfying the first condition. - (B) Cu and Au have the same crystal structure, satisfying the second condition. - (C) The chemical properties of both metals are similar, satisfying the third condition. - (D) The number of valence electrons does not significantly impact the miscibility in this case.

Step 3: Conclusion.
Thus, the correct answer is option (2): (A), (B) and (C) only.