Answer is a) (X∧¬Z)→Y
((refer page 6,7 Discrete Math,ed 7,Kenneth H Rosen))
Implication "P implies Q" i.e (p→Q),where P is premise and Q is Conclusion, can be equivalently expressed in many ways. And the two equivalent expression relevant to the question are as follows
one is " if P then Q "
another is "Q unless ¬P"
both of these are equivalent to the propositional formula (P→Q),
Now compare "If X then Y unless Z" with "Q unless ¬P" , here (¬P = Z) so (P = ¬Z) and (Q = Y)
compare with "if P then Q" , here (P = X) , (Q= Y)
So we get premise P= 'X' and '¬Z' ,conclusion Q = 'Y'
Equivalent propositional formula (X∧¬Z)→Y
EDIT:someone messaged me that i have taken "If X then (Y unless Z)" in above explanation and how to know if we take "(If X then Y) unless Z" or "If X then (Y unless Z)". So let me show both way gives same answer.
"(If X then Y) unless Z" ⇔ "(X→Y)unless Z" ⇔ "¬Z→(X→Y)" ⇔ "¬Z→(¬X ⋁ Y)" ⇔ "Z ⋁ ¬X ⋁ Y"
⇔ "¬(X∧¬Z) ⋁ Y " ⇔ "(X∧¬Z)→Y"