Although Sylvester and John promised to be here, … has turned up.

1. Analyze the question The sentence is: "Although Sylvester and John promised to be here, ...... has turned up." We need the word that correctly completes it. There are two people, and the clause "although... promised" tells us the outcome contradicts the promise, so the blank must be a negative idea (nobody came).

2. Explain the concept

• Both refers to two people, but it takes a plural verb (have) and is positive. "Both has turned up" is wrong in agreement and meaning.
• All refers to three or more, and it takes a plural verb.
• Neither means "not one and not the other" (of two). It takes a singular verb (has) and carries a negative meaning.
• Either means "one or the other". It is not negative by itself, and "either has turned up" does not give the contrast needed after "although".

3. Solution

• Two people are named (Sylvester and John), so we need a word for two: both, neither or either. This rules out all.
• The verb is has turned up (singular), so both is out.
• "Although they promised to come..." suggests neither of them came. Either would need a negative helper (e.g. "hasn't either turned up" is awkward, and the natural form is "neither has turned up").

Completed sentence: "Although Sylvester and John promised to be here, neither has turned up."

4. Answer The correct option is C. neither.

5. Tip Remember the pairing: neither = two people, singular verb, negative meaning. If you see "has/is/was" after the blank and a negative idea, think neither. And remember "all" is never used for just two people.