which do not exist
A) SH6
B) HFO4
C) SI6
D) HClO4
and why?

A) SH6 can not exist ?

S can show Oxidation number from -2 to +6
Lowest oxidation number of S is -2 .
In SF6 its oxidation number is 6 because it F is more electronegative element than S
However H is less electronegative then S so in SH6 is is in -6 Oxidation state which S can not show.That is the reason it can not exist.

B) HFO4 Can not exist
F can show only -1 oxidation state in its compounds and can not have expanded octet duw to absence of d- orbitals
In HFO4 ...F has has +7 oxidation state and an expanded octet which F can not show.Hence HFO4 can not exist.

C) SI6  can not exist as Iodine are are big in size and accommodating 6 big Iodine atoms around S is streically impossible .