A fully isomorphous system needs the two metals to mix smoothly into one another at every composition, in both the liquid and solid states, which only happens when the atoms are close enough in size and the crystal structures line up. Copper and nickel are almost a textbook match for this, both are face centred cubic, their atomic sizes differ by only a couple of percent, and their chemistry is similar enough that one substitutes freely for the other in the lattice. That is why the Cu-Ni phase diagram is just a simple lens shape with liquid, a single solid solution, and a two phase region between them, no intermediate compounds anywhere. Cu-Sn and Ni-Zn form several intermetallic phases instead, and Pb-Sn is a classic eutectic pair with limited mutual solubility. So the isomorphous system here is Cu-Ni, option (A).