The given balanced equation is,

#4 NH_3+5 O_2 -> 4 NO + 6H_2O#

This implies, #4 " moles"# of #NH_3# reacts with #5 " moles"# of #O_2# to produce #4 " moles"# of #NO# and #6 " moles"# of #H_2O#

So, the net products will be, #4 " moles"# of #NO# and #6 " moles"# of #H_2O#

Accordingly, #x " moles"# of #NO# and #4.5 " moles"# of #H_2O# should be formed.

where #x# would be,

#x=(4xx4.5)/6 #

#x=3 " moles"#

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**Note** : The #4:6=> 2:3# ratio remains constant for the given pair of products.

As from the question, if the answer #3 " moles"# is correct, then #3:4.5# must be equal to #2:3#

Checking that, #3:9/2 => 6:9 => 2:3#

therefore, the answer must be correct.

#3 " moles"# of #NO# is formed along with #4.50 " moles" # of #H_2O#.