The author only claims that braille readers will be able to use this new electronic braille system, so whether any other systems can be adapted or not is beyond the scope of the argument. Whether (A) is true or not doesn't affect the strength of the author's reasoning, meaning (A) isn't necessary.

When the author talks about people who have the ability to read in braille, that includes people who read both with braille and with other systems. The author doesn't distinguish people who read with other systems, as long as they can still read in braille, so it's not necessary to assume such a distinction.

This must be true in order for the conclusion to follow. If braille readers didn’t have the ability to distinguish symbols using heat, then they wouldn’t easily adapt to the new system despite the symbols being the same. So negating (C) would break apart the argument, meaning it's necessary to assume (C).

The argument is only about braille; even with the new electronic braille system, we know that the symbols used will match those of traditional braille. The exact symbols used in braille or any other system, alphabet-shaped or otherwise, don't make a difference to the argument.

The conclusion is just about people who already know traditional braille being able to read electronic braille. Whether the learning process will be able to go the other way in the future doesn't affect the argument, so it can't be necessary to assume.