Final answer:
The statement that intermetallic compounds are always brittle is incorrect, as their brittleness varies and is not an absolute characteristic of all such compounds.
Step-by-step explanation:
Concerning intermetallic compounds formed in alloys, the statement that is incorrect is 'c. They are always brittle.' While many intermetallic compounds are known for their brittleness due to the ordered nature of their atomic structure, it is not an absolute rule that applies to all intermetallic compounds. Intermetallic compounds do have a defined stoichiometry, tend to exhibit metallic properties such as conductivity and luster, and they can enhance material properties, making options 'a', 'b', and 'd' correct. For example, certain intermetallic compounds are used specifically for their hardness or heat-resistant properties in various applications.
Metallic solids such as those made of copper, aluminum, and iron are characterized by their 'sea' of delocalized electrons which are responsible for many of their metallic properties, including conductivity and malleability. Transition metals, which are common constituents of alloys, notably form various compounds that may or may not be brittle, challenged by the fact that many transition metals themselves are hard and high-melting solids, but also malleable and ductile.