Remember, the cosine parent function has a period of 2pi. It decreases from zero to pi (half way) and then increases. So choice 1 is good. Choice 2 and 3 have period adjustments 2pi/2=pi, so that will decrease half way (pi/2) and increase the other half. So choice 2 is good also. But choice 3 (2pi/3) will decrease only half of that period which is pi/3 and then increase. Pi/3 comes before pi/2, so it will start to increase at pi/3. Choice 3 is out and if you know what tangent looks like, that's out too.