Joe has a delusional belief. When people argue with him,

a. he admits he could be wrong.
b. he only admits he is wrong after being shown more proof than most people would need.
c. he doesn't admit he is wrong to other people, but he admits it to himself.
d. he doesn't admit he could be wrong, no matter what proof he is shown.

d. he doesn't admit he could be wrong, no matter what proof he is shown.