Question 891769
{{{drawing(300,300,-1.2,1.2,-1.2,1.2,
grid(0),red(circle(0,0,1)),circle(1,0,0.01),
arrow(1.15,0.5,1.015,0.05),locate(0.9,0.65,P(1,0)),
green(arc(0,0,1.6,1.6,270,360)),green(arc(0,0,1.6,1.6,180,270)),
green(arc(0,0,1.6,1.6,90,180)),green(arc(0,0,2,1.6,0,90)),
green(line(0.9,-0.1,1,0)),green(line(1.1,-0.1,1,0)),
locate(-0.56,-0.46,green(2pi))
)}}} The angle is measured counterclockwise from the positive x-axis.
As the point (1,0) is on the positive x-axis, we could say that the angle is zero, but it could also be a whole counterclockwise turn around the circle.
Since the whole circumference measures {{{2pi}}} ,
a whole counterclockwise turn is an angle of  {{{highlight(2pi)}}} radians.
Maybe there is some convention, accepted by many, that says angles corresponding to a point are to be expressed as {{{theta}}} with {{{0<theta<=2pi}}} .
If so, I do not know about it.
We could also say that the angle is 2 counterclockwise turns, {{{4pi}}} , or any number of clockwise  or counterclockwise turns, like {{{1002pi}}} or {{{-12pi}}} .
None of those is a given choice.
{{{pi}}} is an half turn and correspond to point (-1,0) at the left end of the circle.
{{{pi/2=(1/4)*2pi}}} is {{{1/4}}} of a counterclockwise turn, and corresponds to point (0,1) at the top of the circle.
{{{3pi/2=(3/4)*2pi}}} is {{{3/4}}} of a counterclockwise turn, and corresponds to point (0,-1) at the bottom of the circle.