One mole of argon gas will have least density at:
A) STP
B) $0^\circ C$ and $1$ atm
C) $273^\circ C$ and $2$ atm
D) $273^\circ C$ and $1$ atm

Hint: Formula of the density is given by - $D = \dfrac{{PM}}{{RT}}$ [Where, $P$ is the pressure, and $T$ is the absolute temperature]. If the ratio between the pressure and the temperature is minimum, only then the density will be the minimum.

Complete answer:
Formula of the density is given by - $D = \dfrac{{PM}}{{RT}}$
[Where, $D$ is the density, $P$ is the pressure, $M$ is the mass, which will be constant for any cases of argon, $R$ is the universal gas constant, $T$ is the absolute temperature]
So, if the ratio between the pressure and the temperature is minimum, only then the density will be the minimum.
So, by observing the options, we can say that in option D, the temperature is maximum, $273^\circ C$ and the pressure is minimum, $1$ atm.
So, One mole of argon gas will have the least density at $273^\circ C$ and $1$ atm.
This Density depends on the fact that how the atoms or particles are arranged in a substance or matter in a certain, fixed volume. If the atoms are loosely packed together having so many spaces between them, will have a low density and it will cause them to float. And if the atoms are tightly packed together with very little space between them, it will have higher density. And, this arrangement depends on the physical changes, like pressure and temperature.
Hence the correct answer is option D.

Additional information: Argon is a chemically inert gas, which means it is chemically unreactive. It is a group of 18 elements of the periodic table. It is also called a noble gas because it has the noble gas configuration, which means its outermost electron shell or valence shell is completely filled. It is the most stable element due to this reason. So, it does not show any tendency to further gain or lose electrons. For the same reason of having the maximum number of valence electrons in its outer shell, it is placed separately in group 18.

Note:
 It is to be noted that STP means Standard Temperature and Pressure. Which means $0^\circ C$ and $1$ atmospheric pressure. So, it does not give the least density. The noble gas, which has the least density at STP is helium, and not argon.