The best example of an implied power from the options provided is issuing an executive order.
Implied powers are those powers that are not explicitly stated in the Constitution but are necessary for the government to effectively carry out its enumerated powers. The power to issue executive orders falls under the president's role as the chief executive and is derived from the duties assigned to the president, even though it is not explicitly mentioned in the Constitution.
The other options listed (appointing justices, appointing ambassadors, and convening Congress) are explicitly granted to the president by the Constitution.