Concept:
To find the day of a future date, we need the day of some known date and then count the number of odd days between them.
Here, the target date is
\[
31^{st}\text{ December}
\]
Step 1: Check Statement I.
Statement I says that
\[
1^{st}\text{ March was Monday}
\]
From \(1^{st}\) March to \(31^{st}\) December, the number of days difference is fixed for every year because February is already over.
So the leap year information does not affect dates from March to December.
Thus, Statement I alone is enough to determine the day of \(31^{st}\) December.
Step 2: Check Statement II.
Statement II says the year was a leap year.
Knowing only that a year is leap year does not tell the day of \(31^{st}\) December.
For example, different leap years may have different weekdays on \(31^{st}\) December.
So Statement II alone is not sufficient.
Step 3: Final answer.
\[
\boxed{\text{Statement I alone is sufficient}}
\]