find a compound proposition involving the proposition p,q,and r that is true when exactly one of p,q and r istru and is false otherwise.

One possible compound proposition is:

(p ∧ ¬q ∧ ¬r) ∨ (¬p ∧ q ∧ ¬r) ∨ (¬p ∧ ¬q ∧ r)
This proposition is true when exactly one of p, q, and r is true and the others are false. When all three are true or all three are false, the entire proposition is false.