Select the most appropriate meaning of the underlined segment in the given…

An idiom's meaning is fixed by convention and cannot be derived by combining the literal meanings of its individual words. "A tall order" is a standard English idiom meaning a demanding or difficult task or request — something that requires considerable effort to achieve, especially within given constraints such as time.

In the given sentence, Mamtha describes finishing the entire project by Saturday as "a tall order." The follow-up clause, "but we'll do our best to meet the deadline," signals that extra effort will be needed to meet this constraint. So the underlined segment conveys that completing the project by the stated deadline is a difficult task.

• "Task for a common cause" refers to a task undertaken jointly for a shared purpose, not to how demanding a task is — unrelated to the idiom's sense of difficulty.

• "Hidden advantage" refers to an undisclosed benefit or edge, which is not part of the idiom's meaning at all.

• "Top priority" refers to the ranking or urgency assigned to something, not the level of effort required to accomplish it — "a tall order" says nothing about ranking.

• "Difficult task" matches the idiom's standard meaning: something that demands considerable effort to accomplish.

Therefore, the underlined segment "tall order" means a difficult task.