Intelligence, which is based on the accumulation of information, skills, and strategies learned through experience, remains steady and in some cases actually improves.
a. fluid
b. deep
c. conscious
d. crystallized

The answer in the missing blank is letter D. Crystallized intelligence, which is based on the accumulation of information, skills, and strategies learned through experience, remains steady and in some cases actually improves. It is accessed through the long-term memory and it improves through our age.