(i) As per the position in the periodic table, the element A is phosphorus as below:
(ii) Phosphorus reacts with dry chlorine to form phosphorus trichloride as below
\(\underset { (A) }{ { P }_{ 4 } } +6{ Cl }_{ 2 }\longrightarrow \underset { (B) }{ { 4PCl }_{ 3 } } \)
Hence, compound B is phosphorus trichloride.
(iii) PCl₃ on hydrolysis gives phosphorus acid H₃PO₃, which is dibasic acid,
\(\underset { (B) }{ { PCl }_{ 3 } } +3{ H }_{ 2 }O\longrightarrow { 3HCl }+\underset { (C) }{ { H }_{ 3 }{ PO }_{ 3 } } \)  
Hence, compound C is phosphorus acid.
(iv) H₃PO₃ on heating undergo auto oxidation and reduction producing phosphoric acid md phosphine as below:
\(\underset { (c) }{ { 4H }_{ 3 }{ PO }_{ 3 } } \overset { \Delta }{ \longrightarrow } { PH }_{ 3 }+\underset { (b) }{ { 3H }_{ 3 }{ PO }_{ 4 } } \)
Phosphoric acid is a tribasic acid. Hence, compound D is phosphoric acid.
Thus A = Phosphorus
B = Phosphorus trichloride
C = Phosphorus acid
D = Phosphoric acid

(i) The element which occupies group number 15 and period number 3 is phosphorus. Therefore A is phosphorus. Phosphorus reacts with chlorine to give PCI₃There fore compound B is phosphorus trichloride and it has a pyramidal shape.
P₄ + 6Cl₂ \(\longrightarrow \) 4PCl₃
PCl₃ further reacts with Cl₂ to give PCl₅
Therefore, the compound C is phosphorus pentachloride and it has a trigonal bipyramidal shape.
PCI + CI\(\longrightarrow \) PCI
Thus, A= Phosphorus
B = Phosphorus trichloride (PCI₃)
C = Phosphorous pentachloride (PCI₅)

(i) The element which occupies group number 15 and period number 3 is phosphorus. Hence, A is phosphorus This is further supported by the fact that A exhibits allotropy and A is tetra atomic.
(ii) Phosphorus reacts with caustic soda to form phosphine as below:
4P + 3 NaOH + 3H₂O \(\longrightarrow \) PH₃ + 3 NaH₂PO₂
Hence, B is phosphine PH₃ which has rotten fish odour.
(iii) Phosphorus reacts with dry chlorine to form phosphorus trichloride which has a pungent odour.
P₄ + 6 Cl₂\(\longrightarrow \) 4PCl₃
Thus, A = Phosphorus
B = Phosphine
C = Phosphorus trichloride

(i) PCl₅ + 2Ag \(\longrightarrow \) 2 AgCl + PCl₃
(ii) AgCI_(s)+ 2NH_3(aq) \(\longrightarrow \)[Ag(NH₃)₂]CI_(aq)
(iii) PCl₃ + 3H₂O \(\longrightarrow \) H₃PO₃ + 3HCI
(iv) PCl₅ + H₂O \(\longrightarrow \) POCl₃ + 2HCI
(v) NaH₂PO₂ + HCI \(\longrightarrow \) H₃PO₂ + NaCl

Properties of inter halogen compounds:
(i) The central atom will be the larger one.
(ii) It can be formed only between two halogen and not more than two halogens.
(iii) Fluorine can't act as a central metal atom being the smallest one.
(iv) Due to high electro negativity with small size fluorine helps the central atom to attain high coordination number.
(v) They can undergo the auto ionization
\(2ICI\rightleftharpoons { 1 }^{ + }{ ICI }_{ 2 }^{ - }\)
\(2ICI_3\rightleftharpoons { ICI_2 ^+ }+{ ICI }_{ 4 }^{ - }\)