Identify the correct decreasing order of lattice enthalpy for metal...

Lattice enthalpy is directly proportional to the product of charges of the ions and inversely proportional to the internuclear distance between them.

$U \propto \dfrac{q_1 q_2}{r_+ + r_-}$

For alkali metal fluorides, the charges on the ions are the same (+1 and -1). The ionic radius of alkali metal cations increases down the group in the order: $\text{Li}^+ < \text{Na}^+ < \text{K}^+$.

As the size of the cation increases, the internuclear distance increases, which leads to a decrease in the lattice enthalpy.

Therefore, the decreasing order of lattice enthalpy is $\text{LiF} > \text{NaF} > \text{KF}$.

Answer: $\text{LiF} > \text{NaF} > \text{KF}$