Mesh Error due to Patch Conformal
 

I got some Mesh Error,
I am newly to Ansys, expecting your someone can assist

a. I am not sure what the Question Mark stands for in the geometry section. ?
b. Why it shows the "Patch-conforming tetrahedral mesh failed because of duplicated nodes in boundary mesh". I didn't understand how I can tackle this problem. Using Patch Independent, I am still getting error but not prefer to use since patch independent doesn't cover small edges properly
c. what is the topology check do we need to do it before every meshing?
d. what is defeaturing means. ?

Thanks