Answer:- Octet of NO is incomplete.
Explanations:- For sulfur trioxide, there are total 24 valence electrons. S is the central atom. Two O atoms are bonded to S via single bonds and the third O is bonded to S via double bond. So, in this way, the octet of each O is complete. Total 8 electrons are used in bonding. Each of the single bonded O has three lone pairs and the double bonded O has two lone pairs. It means total 16 electrons are present in the form of lone pairs. The sum of electrons used in bonding and present as lone pairs is 16+8 = 24 and this is exactly same as total valence electrons. So, the octet of S is also complete.
hydrogen sulfide has total 8 valence electrons. H has only one valence electron and can form only a single bond. So, two H atoms are bonded to S via single bonds. Total 4 electrons are used in bonding and remaining 4 electrons are present as two lone pairs on S. So, the octet of S is also complete.
Phosphane has total 8 valence electrons. P is the central atom and the three H atoms are bonded to it via single bonds. Total 6 electrons are used in bonding and rest of the two electrons are present as one lone pair on P. The octet of P is also complete.
NO has 11 valence electrons. Either we make a single, double or triple bond between N and O, the octet for at least one of them is incomplete as there are add number of electrons. The best lewis structure of NO has a triple bond between N and O, 4 electrons around it and N has 3 electrons around it. The octet of O is complete but the octet of N is incomplete as it could complete only 7 electrons.
So, the right answer is NO.
Lewis structures for all the four molecules are given below:
