Uranium235
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Message - first 4 bits.
In the 5th bit, we write 0 if the first 2 bits are 00 or 01, and 1 if they are 10 or 11.
In the 6th bit, we write 0 if the 3rd and 4th bits are 00 or 01, and 1 if they are 10 or 11.
In the 7th bit, we write 0 if the 5th and 6th bits are 00 or 01, and 1 if they are 10 or 11.
Let's say the message is 1001. We append - 1001101.
If nothing is corrupted, then everything matches.
If the first bit of the message is corrupted, it will be visible because it won't match the 5th bit. At the same time, the 5th bit matches the 7th bit.
And if the 5th bit is corrupted, it won't match the 7th bit. And also not with the first and second.
If the 7th bit is corrupted, it won't match the 5th and 6th bits.
If the 6th bit is corrupted, it won't match the 3rd and 4th and 7th bits.
....
In general, I haven't checked all the options, but there's a chance that this is correct; I couldn't come up with anything better.
In the 5th bit, we write 0 if the first 2 bits are 00 or 01, and 1 if they are 10 or 11.
In the 6th bit, we write 0 if the 3rd and 4th bits are 00 or 01, and 1 if they are 10 or 11.
In the 7th bit, we write 0 if the 5th and 6th bits are 00 or 01, and 1 if they are 10 or 11.
Let's say the message is 1001. We append - 1001101.
If nothing is corrupted, then everything matches.
If the first bit of the message is corrupted, it will be visible because it won't match the 5th bit. At the same time, the 5th bit matches the 7th bit.
And if the 5th bit is corrupted, it won't match the 7th bit. And also not with the first and second.
If the 7th bit is corrupted, it won't match the 5th and 6th bits.
If the 6th bit is corrupted, it won't match the 3rd and 4th and 7th bits.
....
In general, I haven't checked all the options, but there's a chance that this is correct; I couldn't come up with anything better.