Recall what electromagnetic waves are

Electromagnetic waves are transverse waves made of oscillating electric and magnetic fields. Examples include:

$\text{radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays}.$

Check each option

A: Cosmic rays

• In standard school/JEE context, cosmic rays are primarily high-energy charged particles coming from outer space, mainly protons and atomic nuclei.
• Hence, they are not electromagnetic waves.

B: Gamma rays

• Gamma rays are part of the electromagnetic spectrum.
• So they are electromagnetic waves.

C: $\beta$-rays

• $\beta$-rays are streams of fast electrons or positrons emitted in radioactive decay.
• These are particles, not electromagnetic waves.

D: $X$-rays

• X-rays are also part of the electromagnetic spectrum.
• So they are electromagnetic waves.

Conclusion

The options that are not electromagnetic waves are:

$A \text{ and } C$

Comparison with stored answer

• Stored correct answer: C
• Derived answer: A, C

Therefore, I do not agree with the stored answer. If this were intended as a single-correct MCQ, the question is flawed because both cosmic rays and $\beta$-rays are not electromagnetic waves.