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 $(X∧¬Z)→Y$
PS: 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 that both way gives the same answer.
"(If $X$ then $Y$) unless $Z$" $⇔(X→Y)$ unless $Z$
$\quad \quad ⇔ ¬Z→(X→Y)$
$\quad \quad⇔¬Z→(¬X \vee Y)$
$\quad \quad⇔Z \vee ¬X \vee Y$
$\quad \quad⇔ ¬(X∧¬Z) \vee Y $
$\quad \quad⇔ (X∧¬Z)→Y$