Which of the following commands will reduce all consecutive spaces down to a single space?

tr interprets its SET1 argument as a list of literal characters, not a regex, so '\s' matches a literal backslash and the letter s rather than whitespace - this does not squeeze spaces.

The -c flag complements SET1, so -c ' ' applies the operation to every character that is NOT a space - this would squeeze repeated non-space characters instead.

The -d flag deletes every occurrence of the specified character entirely, so tr -d ' ' removes all spaces from the output rather than reducing consecutive ones to a single space.

The -r flag is not defined in POSIX or GNU tr and causes an error on standard systems; reversing the operand order is done by swapping SET1 and SET2 arguments, not with a -r flag.

The -s (squeeze-repeats) flag instructs tr to replace each run of consecutive identical characters listed in SET1 with exactly one instance of that character. Running tr -s ' ' reads from a.txt and writes to b.txt, collapsing any sequence of two or more adjacent spaces into a single space while leaving all other characters untouched.