What is Quantum gate?

Answer: A. A superposition that gives 0 and 1 with 50% probability each when measured; the position on the equator (the longitude) represents the relative phase between |0⟩ and |1⟩

The Bloch sphere represents a single-qubit state cos(θ/2)|0⟩ + e^{iφ} sin(θ/2)|1⟩ as a point on a sphere, with the angle θ playing the role of latitude and φ that of longitude. The north pole is |0⟩ and the south pole is |1⟩; on the equator (θ = 90°) the amplitudes of |0⟩ and |1⟩ have equal magnitude, so a measurement gives 0 and 1 with 50% probability each. Points on the equator differ in the relative phase φ: (|0⟩ + |1⟩)/√2 and (|0⟩ − |1⟩)/√2 have the same measurement probabilities but are different states. Quantum gate operations can be represented as rotations of this sphere.