Answer: Option (a) is the correct answer.
Step-by-step explanation:
The elements cobalt (Co), copper (Cu), and Scandium (Sc) are all d-block elements and are also known as transition elements. Thus, all these show similar properties as all of them are transition elements.
Whereas Actinium (Ac) is a lanthanide series or f-block element. It shows different properties as compared to transition elements.
Thus, we can conclude that out of the given options, Ac has properties different than the rest.