Answer:
The smallest unit which maintains the physical properties of a compound is a Molecule.
Step-by-step explanation:
Option-A, Atom is incorrect because statement is asking about the physical properties of a compound. Therefore, as compound is made up of two or more, same or different atoms hence, atom can not be the correct answer.
Option-B, Element is also incorrect because Element can be mono-atomic like He, Ne, Ar e.t.c or a molecules like N₂, O₂, F₂ e.t.c but it can't be a compound because in compound the joining elements are different as all compounds are molecules but all molecules are not compounds.
Option-C, Molecule is the correct answer because all compounds are molecules unless the atoms forming the compound are different.
Option-D, Alloy is also incorrect because Alloys are a homogeneous mixture of two or more metals. Hence, they can't be considered as compounds as they can be separated by physical means.