Question

A monatomic gas is,1.oxygen 2.Argon 3.hydrogen sulphide 4.Ammonia

Answer 1

Its 2.) Argon the ans.

Answer 2

The correct answer is:  2)  "argon" ;  or,  " Ar " ; 
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                                    → {which is the corresponding symbol for "argon"                                            in the "Periodic Table of Elements"}.
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Explanation:
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Note that among the answer choices provided;  
Answer choice:  " 3)  "hydrogen sulfide" {sic} is misspelled and is supposed to be:    " 3)  hydrogen sulfide " . 
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Note that "argon" {" Ar " } is one single element.  "Argon"  is one of the 6 (six) "noble gases" , which are all "monatomic" as a gas;  and are:
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         1)  helium (He),    2)  neon (Ne),    3)  argon (Ar),    4)  krypton (Kr),   5) xenon,  and:  6)  radon (Rn) .
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Let us review the other answer choice provided to see if they are incorrect:
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Choice 1)  "oxygen" ; is INCORRECT.  "Oxygen" exists in gaseous form as either:  " O₂ " —"oxygen gas"—which is "diatomic" [has 2 (two) atoms of oxygen per molecule—and as such, NOT "monatomic" ; 
   or:  " O₃ " — "ozone gas" —which is "triatomic" [has 3 (three) atoms of oxygen per molecule—and as such, NOT "monatomic". 
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Choice 3)  "hydrogen sulfide" {Note the correct spelling:  "sulfide"} — is INCORRECT.  "Hydrogen sulfide" ;  that is:  " H₂ S " — is a gaseous compound that has 2 (two) atoms of hydrogen per molecule AND 1 (one) atom of sufur per molecule—with a total of 3 (three) atoms per molecule.  Thus, "hydrogen sulfide" is "triatomic" —and as such, NOT "monatomic". 
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Choice 4)  "Ammonia" is INCORRECT.  "Ammonia" ; that is:  "NH ₃ " — exists as a gaseous compound that has 1 (one) atom of nitrogen per molecule AND 3 (three) atoms of hydrogen per molecule—with a total of 4 (four) atoms per molecule.  Thus, "ammonia" is "polyatomic" —more specficially, "tetratomic"—and such, NOT "monoatomic".
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Shortcut tip:  We can actually rule out Choices 3 and 4 really easily because they are "compounds" (consist of more than one element), which means, at the minimum, they would be "diatomic or having an even greater number of atoms"; and as such:  NOT "monatomic" .
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Hope this answer—and explanation—are of help to you!
Best wishes!
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