1. Principle involved

When a substance is placed in a non-uniform magnetic field:

• Paramagnetic substances are attracted toward the stronger magnetic field.
• Diamagnetic substances are repelled from the stronger magnetic field.

From the figure description, the magnet is placed above the pan. Therefore:

• If the substance is attracted upward toward the magnet, the pan moves upwards.
• If the substance is repelled downward away from the magnet, the pan moves downwards.

So:

• Paramagnetic $\Rightarrow$ pan deflects upwards
• Diamagnetic $\Rightarrow$ pan deflects downwards

2. Check each option

Option A: $H_2O(l)$

Water is diamagnetic because all electrons are paired.

Hence it is repelled by the magnetic field, so the pan should move downwards, not upwards.

Therefore, A is false.

Option B: $K_4[Fe(CN)_6](s)$

In $[Fe(CN)_6]^{4-}$:

• Oxidation state of Fe: $x + 6(-1) = -4 \Rightarrow x=+2$
• So Fe is $Fe^{2+}$, i.e. $3d^6$.
• $CN^-$ is a strong field ligand, so the complex is low spin: $t_{2g}^6 e_g^0$

All electrons are paired, so the complex is diamagnetic.

Hence the pan should move downwards, not upwards.

Therefore, B is false.

Option C: $O_2(g)$

Molecular oxygen has two unpaired electrons in antibonding $\pi^*$ orbitals, so it is paramagnetic.

Therefore it is attracted toward the magnetic field and the pan moves upwards, not downwards.

Therefore, C is false.

Option D: $C_6H_6(l)$

Benzene has all electrons paired, so it is diamagnetic.

Hence it is repelled from the magnetic field and the pan moves downwards.

Therefore, D is true.

3. Final derived answer

The correct option is:

$\boxed{D}$

4. Comparison with stored correct answer

Stored correct answer: $A, B, C$

My derived answer is $D$ only. So I disagree with the stored answer.

The likely issue is that the direction of deflection has been interpreted oppositely in the stored answer. Using the standard rule that a magnet above attracts paramagnetic substances upward and repels diamagnetic substances downward, only benzene gives downward deflection.