<h3>Answer:</h3>
The smallest unit which maintains the physical properties of a compound is a <u>Molecule</u>.
<h3>Explanation:</h3>
Option-A, <u><em>Atom</em></u> 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, <u><em>Element</em></u> is also incorrect because Element can be mono-atomic like He, Ne, Ar <em>e.t.c </em>or a molecules like N₂, O₂, F₂ <em>e.t.c</em> 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, <u><em>Molecule</em></u> is the correct answer because all compounds are molecules unless the atoms forming the compound are different.
Option-D, <em><u>Alloy</u></em> 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.