1. We check each option against the stated periodic/property trend.

2. Option A:

Given: $CO_2 < SiO_2 < SnO_2 < PbO_2$ for increasing oxidising power.

• In group 14, the stability of the $+4$ oxidation state decreases down the group due to inert pair effect.
• Hence, dioxides of heavier elements more readily get reduced from $+4$ to $+2$, so they act as stronger oxidising agents.
• Thus oxidising power increases down the group: $CO_2 < SiO_2 < SnO_2 < PbO_2$

So, A is correct.

3. Option B:

Given: $HF < HCl < HBr < HI$ for increasing acid strength.

For hydrogen halides in aqueous medium, acid strength increases down the group because the H--X bond strength decreases: $HF < HCl < HBr < HI$

So, B is correct.

4. Option C:

Given: $NH_3 < PH_3 < AsH_3 < SbH_3$ for increasing basic strength.

Actual trend for group 15 hydrides:

• Basic strength decreases down the group.
• Reason: availability of lone pair decreases and atomic size increases.

Thus, $NH_3 > PH_3 > AsH_3 > SbH_3$

So the given order is wrong.

Hence, C is not strictly according to increasing basic strength.

5. Option D:

Given: $B < C < O < N$ for increasing first ionization enthalpy.

Across period 2, ionization enthalpy generally increases, with two important exceptions:

• $B < Be$
• $O < N$

Among the given elements: $B < C < O < N$ This is correct because nitrogen has extra stability due to half-filled $2p^3$ configuration, so: $IE_1(B) < IE_1(C) < IE_1(O) < IE_1(N)$

So, D is correct.

6. Therefore, the sequence that is not according to the stated property is: $\boxed{C}$