When the temperature of a gas is increased, it is either accompanied by an increase in pressure if the gas is enclosed in a container, or an increase in volume if the gas is able to expand.

a) same
b) When the temperature of a gas is increased, it is accompanied either by an increase in pressure if it is enclosed in a container or
c) .....
d) Any increase in the temperature of a gas is either accompanied by an increase in pressure if it is enclosed in a container, or by
e) Any increase in the temperature of a gas is accompanied either by an increase in pressure if the gas is enclosed in a container or by

e is correct.

Now in this example:
Is it true that b) is wrong because it did not end in "by" ?
Why is d) wrong because of ","
Again my question is does it matter if the sequence is "accompanied either" Or "either accompanied"

Yes, the sequence of the words matters. "Either X or Y" is an idiom. Whatever follows "either" (represented by X) must be parallel to whatever follows "or" (represented by Y).

So if I say "either accompanied by cheetos or..." I have to say "or accompanied by fritos." X = accompanied by cheetos, Y = accompanied by fritos
If I say "accompanied either by cheetos or..." now I just have to say "or by fritos." X = by cheetos, Y = by fritos

B does not repeat the second "by" and D does not repeat the "accompanied."