In the figure above, triangle MNO is equilateral. What is the area of

(D) is our answer.

There are two different approaches to solving this question.
A Precise approach relies on knowledge of geometric rules:
(1) NP is a leg in a 30-60-90 triangle NPO meaning we can calculate the hypotenuse (which is the side of the triangle). Then we have the height and base of a triangle and can calculate its area.
(2) a useful equation to remember is that the area of an equilateral triangle is \(\frac{a^2\sqrt{3}}{4}\), where 'a' is the length of a side.
(Remember 1-2-3-4 i.e. length - square - sqrt(3) - div by 4)

Then both are sufficient.

A second approach, which does not rely on knowledge of geometric rules is Alternative.
An equilateral triangle is a regular polygon and there is only ONE way to draw a regular polygon.
That is - once you know the length of just one of the sides/heights/chords/etc. you know all of the shape's properties. (Try it out - you can SEE this)
So both (1) and (2) are sufficient.