Explanation:

In mathematics (and more specifically geometry), a semicircle is a one-dimensional locus of points that forms half of a circle. The full arc of a semicircle always measures 180° (equivalently, π radians, or a half-turn). It has only one line of symmetry (reflection symmetry).
Angle in a semicircle = 1800 = π
Hence, option 1 is the correct answer.