In which one of the following arrangements the given sequence is not strictly according to the properties indicated against it ?

The correct option that is not strictly according to the indicated property is:
H₂O < H₂S < H₂Se < H₂Te : Increasing pK_a values

To understand why this sequence is incorrect, let us analyze the trends in acidic strength and pK_a values for the group 16 hydrides (H₂O, H₂S, H₂Se, and H₂Te):

1. Acidic Strength Trend:
As we move down the group from oxygen (O) to tellurium (Te), the size of the central atom increases. The atomic sizes follow the order:
O < S < Se < Te
Due to the increasing size of the central atom, the H-E bond length (where E is the group 16 element) increases, which significantly weakens the bond. The bond dissociation enthalpy decreases in the order:
H-O > H-S > H-Se > H-Te
Since the H-E bond becomes weaker down the group, it becomes increasingly easier to release a proton ($H^{+}$). Therefore, the acidic strength increases down the group:
H₂O < H₂S < H₂Se < H₂Te (Increasing acidic strength)

2. Relation between Acidic Strength and pK_a:
The acid dissociation constant ($K_a$) is directly proportional to acidic strength. The relationship between pK_a and $K_a$ is given by the formula:

${\text{pK}}_{\text{a}}=-{\log }_{10}{K}_a$

This negative logarithmic relationship means that a stronger acid (which has a larger $K_a$) will have a smaller pK_a value.
Since acidic strength increases from H₂O to H₂Te, the values of $K_a$ increase, which means the pK_a values must decrease.

Therefore, the correct increasing order of pK_a values is:
H₂Te < H₂Se < H₂S < H₂O
Thus, the given sequence H₂O < H₂S < H₂Se < H₂Te is actually the order of decreasing pK_a values (or increasing acidic strength), making this arrangement incorrect.