The source of electromagnetic waves can be a charge:

(a) moving with a constant velocity
(b) moving in a circular orbit
(c) at rest
(d) falling in an electric field

1. (b, d)
2. (a, c)
3. (b, c)
4. (c, d)

(b, d) Hint: The electromagnetic wave is produced by an accelerating charge.
Step 1: Find the situations in which the charge is accelerating.
Here, in option (b) charge is moving in a circular orbit.
In a circular motion, the direction of the motion of the charge is changing continuously, thus it is an accelerated motion and this option is correct.
Also, we know that a charge starts accelerating when it falls in an electric field.