Wrong statement :- (3) Magnetic field is along x-axis
(1) Magnetic field along y-axis
Given velocity vector and acceleration is in x-z plane .
Hence Magnetic field along y-axis makes the charged particle to move in x-z plane and
gives centripetal acceleration in x-z plane. This statement is true
(2) a1b1 + a2b2 = 0
This indicates acceleration and velocity are in perpendicular direction.
Magnetic field will not change the speed only makes it to move in circular motion.
Hence velocity and centripetal acceleration are mutually perpendicule . Hence this statement is true.
(3) Magnetic field is along x-axis
Given velocity is in x-z plane. If magnetic field is along x-axis, velocity can not be in x-z plane
Hence this is wrong statement
(4) Kinetic energy is constant :- true, magnetic field will not change the speed of charge particle