Recall the form of a theory-based confidence interval for a population proportion is p multiplier

Recall the form of a theory-based confidence interval for a population proportion π is 

p̂ ± multiplier × √p̂ (1 − p̂) / n.

a. Remind me: What’s the difference between p̂ and π?

b. The midpoint of this interval is always equal to what?

c. What is √p̂ (1 − p̂ / n called?

d. What is multiplier × √p̂ (1 − p̂ / n called?