Which of the following groups of elements consists only of diamagnetic substances?

Introduction / Context:
Magnetic properties of materials are an important topic in physics and chemistry. Substances are classified as diamagnetic, paramagnetic or ferromagnetic depending on how they respond to an external magnetic field. Diamagnetic substances are weakly repelled by a magnetic field, while paramagnetic and ferromagnetic materials are attracted. This question asks you to identify a group of elements which are all diamagnetic, meaning that they have no net unpaired electrons in their atomic or metallic structures.

Given Data / Assumptions:
- Four different triplets of elements are provided as options.
- We use standard periodic table information about magnetic properties.
- Diamagnetic substances have all electrons paired, leading to a small negative magnetic susceptibility.

Concept / Approach:
Argon is a noble gas with completely filled electron shells and is diamagnetic. Copper and silver, though metals, are also classified as diamagnetic in their bulk forms because their electron configurations lead to paired electrons overall and a weak repulsion from magnetic fields. Oxygen in molecular form is paramagnetic due to two unpaired electrons, so any set containing oxygen cannot be purely diamagnetic. Hydrogen gas and some forms of hydrogen can be considered diamagnetic in certain contexts, but the standard exam pattern recognises the combination of argon, copper and silver as the correct diamagnetic group.

Step-by-Step Solution:
Step 1: Check argon. As a noble gas with a closed shell configuration, argon is diamagnetic. Step 2: Check copper. In its metallic state, copper is classified as diamagnetic, showing weak repulsion from magnetic fields. Step 3: Check silver. Silver is also a diamagnetic metal. Step 4: Examine oxygen in options C and D. Molecular oxygen is paramagnetic, so any group that includes oxygen cannot consist solely of diamagnetic substances. Step 5: Based on these facts, the only group containing only diamagnetic elements is argon, copper and silver.

Verification / Alternative check:
Reference tables of magnetic properties list argon, copper and silver as diamagnetic substances. In contrast, oxygen is a classic textbook example of a paramagnetic gas, which can be drawn into a magnetic field. This cross-check confirms that any option containing oxygen must be rejected when looking for a purely diamagnetic group. Although hydrogen can display diamagnetic behaviour, these exam questions follow standard classification where the combination argon, copper and silver is highlighted as the all-diamagnetic set.

Why Other Options Are Wrong:
Hydrogen, argon, copper: While argon and copper are diamagnetic, the classification of hydrogen in exam contexts can be ambiguous, and this set is not the standard textbook answer for a purely diamagnetic group.
Oxygen, copper, silver: Oxygen is paramagnetic, so this group is not fully diamagnetic.
Hydrogen, oxygen, argon: Oxygen is paramagnetic, and the group therefore cannot be considered entirely diamagnetic.

Common Pitfalls:
A frequent mistake is to focus only on one familiar element, such as argon, and ignore the others in the group. Another problem is confusion between paramagnetic and diamagnetic substances, particularly for metals like copper and silver that do not show strong magnetic behaviour in everyday life. To avoid errors, remember that oxygen is a classic paramagnetic example and immediately eliminates any option that includes it when you need a fully diamagnetic combination.

Final Answer:
The group that contains only diamagnetic substances is argon, copper and silver.