Which property of halogen acids indicated below is incorrect?
A. HF > HCl > HBr > HI (acidic strength)
B. HI > HBr > HCl > HF (reducing strength)
C. HI > HBr > HCl > HF (bond length)
D. HF> HCl > HBr > HI (thermal stability)

Hint: Take into consideration each of the properties mentioned here and predict the periodic trend that is observed in that particular group. Take into account various factors like atomic size, tendency of the atom to lose electrons, etc.

Complete answer:
We can check whether the options are correct or not one by one.
In the first case, they are given the order of acidic strength of halogen acids. We know that a good acid is one which can donate hydrogen easily. Also, if the conjugate base formed after the release of hydrogen ion is more stable, then it will be a good acid. In this case, since the size of Iodine is large, it can disperse the negative charge on it as a conjugate base. Hence it can donate hydrogen easily. So, HI will be the strong acid. Also, we know that Fluorine is very small, and it cannot possess a negative charge after donation of hydrogen. So, the first order is incorrect. Because, the HI will be a strong acid and HF will be a weak acid.
Reducing strength refers to the ability of a molecule to accept a lone pair of electrons. Since, iodine is a large molecule and contains many empty orbitals, it can easily accept a lone pair of electrons. On the other hand, fluorine does not have any free orbitals and will be a very bad reducing agent. So the reducing strength will increase down the group and the given order I correct.
A larger atomic and ionic size indicates a larger bond length to accommodate all the orbitals of the atoms. The size of the atoms increases down the group due to addition of new orbitals. Thus, bond length will also increase. The given order is correct.
For thermal stability, the bond has to be strong and should require high energy for the bond to dissociate. The larger the bond length, the farther apart the nuclei of the atoms are. This will result in weaker interatomic forces and will decrease the energy required to break the bonds. Thus, as the bond length increases, thermal stability will reduce. The given order is correct.
Hence, the correct answer to this question is ‘A. HF > HCl > HBr > HI (acidic strength)’

Note: Remember that the basic knowledge of how the atomic size changes along a group and down a period. The trend in almost every other property can be predicted using this knowledge. Smaller atoms are less likely to lose electrons than the larger atoms in the same group but they are also less likely to accept electrons.