Points to note:
1. The magnetic force Bqv = mv2/r is used. Remember to cancel the v and derive the r = mv/Bq. After obtaining r, the period T can be found = 2 pi r /v. Since the charge particle only completed a semicircle , the time taken is half a period.
3. The electric field produces force on charge that would cause acceleration or deceleration . Hence the velocity cannot be constant. Statement 1 cannot be true.
The magnetic force is zero when the dir of B is parallel to the velocity of charge. Hence constant velocity is possible as acceleration is zero.
The electric field and magnetic field when aligned at right angles would produce forces in opposite direction. When the magnitude of this two forces are equal , the resultant force is zero , hence constant velocity is possible as acceleration is zero. A typical arrangement would be E vertically downwards and B into the page. It is true for both positive and negative charges.