The correct option is (A) 2.5 m/s
Explanation:
Since,
v = fλ ---- (1)
where v = speed of the wave
f = frequency of the wave = 5 Hz
λ = wavelength of the wave = 0.5
Plug in the value in (1):
(1) => v = 5 * 0.5
v = 2.5 m/s
Can u go into more detail ?
Explanation:
The moment of A about B is given by the formula

where M is the moment, F is the force acting perpandicular to the axis of rotation about B by A, and d is the horizontal distance of A from B along the axis of rotation.
Now

(a) At X,

(b) At 0.5m from X,

(c) At Y,

where negative sign indicates clockwise rotation of the child about Z.
(d) At Z,

The only item on that list that's a plane is the surface of a flat table. The edge of a desk is a rectangle, square, circle, etc. The intersection of two walls is a part of a straight line. A curve in a road is a part of a circle.
What's going on is that the water in the straw is pushed into the straw by the air pressure outside of the straw. As long as the pressure outside is able to overcome the force of gravity, the liquid will stay in the straw.