Now we will check each option.
1) KEEP →
The above word cannot be formed because the
original word contains only one ‘E’
.
2) BLACK →
The above word cannot be formed because the original word does not contain the letter ‘L’.
3) SKIP →
The above word cannot be formed because the original word does not contain the letter ‘I’.
4) SACK→
The above word can be formed using the original word BACKSPACE.
Hence, SACK is the correct answer.
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