Talking about what might be a on with your study group in not good idea everyone will think different topics are

different                                     34567856yuh                                              

Step-by-step explanation:

That's a cosine wave, which normally has a period of 2π.  One period of the cosine wave is one revolution, or one "up-down-up" of the wave.  It looks like we have 2 revolutions in the length of 2π in our graph.  So the period is 2.  Mathematically, that is solved in the following way.  The function could be rewritten as

y = cos(bx + n) where b is the coefficient of x.  Since the normal period for a cosine function is 2π, we can find the period of this function by using the formula:

which, for us is

which equals π.

That means that one "up-down-up" of the wave occurs in the length of pi instead of 2π.  The period of a trig function is the length of one revolution.  

Since we have one revolution occuring within the length of π, the equation for this graph is choice A, which you have already chosen.  Rest assured, you are correct!