• Assertion (A) is True: A standard Neutron Moisture Meter probe contains a radioactive neutron source, typically an americium-beryllium ($^{241}\text{Am-Be}$) or radium-beryllium ($^{226}\text{Ra-Be}$) pellet.
• Reason (R) is True: The source emits fast (high-energy, $\approx 4.5\text{ MeV}$) neutrons that collide elastically with atomic nuclei in the soil; because the mass of a hydrogen nucleus (proton) is nearly equal to that of a neutron, hydrogen slows down fast neutrons into thermal (slow) neutrons far more efficiently than any other soil element. Since soil water is the overwhelming source of hydrogen, the thermal neutron flux detected by the probe is directly proportional to volumetric soil water content ($\theta_v$).
• Relationship: Because elastic collisions with water's hydrogen nuclei moderate fast neutrons into detectable slow neutrons, Reason (R) provides the exact physical explanation for Assertion (A).
• Conclusion: Both (A) and (R) are true and (R) is the correct explanation of (A) (Option 1).

Correct option: (1) Both (A) and (R) are true and (R) is the correct explanation of (A)