The correct option(s) related to adsorption processes is(are)

Adsorption is broadly classified into physisorption (physical adsorption) and chemisorption (chemical adsorption). Both kinds differ in the nature of the adsorbate-adsorbent interaction, the heat of adsorption, and the number of layers that can be formed.

Case A: Statement - “Chemisorption results in a unimolecular layer.”
Chemisorption involves the formation of chemical bonds (ionic or covalent) between the adsorbate molecules and the surface atoms of the adsorbent. Once every surface atom has formed one bond, no further molecules can be accommodated above the first layer because the force of attraction does not extend appreciably beyond the first chemisorbed layer. Hence chemisorption is always a monolayer (unimolecular layer) process.
Statement A is correct.

Case B: Statement - “The enthalpy change during physisorption is in the range of 100 - 140 kJ mol$$^{-1}$$.”
Physisorption is governed by weak van der Waals forces. The heat of adsorption (enthalpy change) is therefore low, typically $$20{-}40\ \text{kJ mol}^{-1}$$. A value of $$100{-}140\ \text{kJ mol}^{-1}$$ pertains to chemisorption, not physisorption.
Statement B is incorrect.

Case C: Statement - “Chemisorption is an endothermic process.”
Formation of chemical bonds releases energy, so the overall enthalpy change for chemisorption is negative (exothermic). Although an activation energy may be required to initiate bonding, the net process liberates heat.
Statement C is incorrect.

Case D: Statement - “Lowering the temperature favors physisorption processes.”
Physisorption is exothermic. According to Le Châtelier’s principle, decreasing temperature shifts an exothermic equilibrium towards the product side, i.e., more adsorption occurs. Therefore a lower temperature increases the extent of physisorption (until the gas condenses).
Statement D is correct.

Hence the correct choices are:
Option A (Chemisorption results in a unimolecular layer.)
Option D (Lowering the temperature favors physisorption processes.)

Option A, Option D