I) All flowers are pink or have a stem.
(There is at least one flower which isn't pink and doesn't have a stem?)
II) All flowers are either pink or have a stem (exclusive or)
(isn't logical equivalence the negation of the exclusive or?, so would it then be A flower is pink iff it has a stem?)
I have posted my thoughts regarding the solutions, would be thankful for advice and correction if needed.
"Exclusive or" $(p\oplus q)$ can be written as $(p\land\lnot q)\lor(\lnot p\land q)$, where as "if and only if" $(p\leftrightarrow q)$ can be written as $(p\land q)\lor(\lnot p\land\lnot q)$.
And so, it can be seen that $(p\oplus q)$ is equivalent to $(p\leftrightarrow\lnot q)$.
And so $\neg(p\oplus q)$ is equivalent to $(p\leftrightarrow q)$.
[But, be careful using a biconditional in a existential statement.]