In the following question, a sentence is given with a bracketed part that may contain an error. Choose the alternative that will best replace the bracketed part. In case no improvement is needed, select the option no improvement. All (was surprising) to find that he was not with them.

Introduction / Context:
This grammar question tests subject verb agreement and correct tense in a sentence describing a past event. The subject of the sentence is All, which refers to a group of people. The bracketed verb phrase was surprising is grammatically awkward and does not agree in number with the subject. The task is to choose the correct verb form that agrees with All and expresses the intended meaning that everyone experienced surprise on discovering that he was not with them.

Given Data / Assumptions:

• Original sentence: All (was surprising) to find that he was not with them.
• Options: was surprised, were surprised, is surprised, no improvement.
• The context clearly refers to a completed action in the past.
• All refers to more than one person, functioning as a plural subject.

Concept / Approach:
In English, subject verb agreement requires that plural subjects take plural verbs in simple tenses. The indefinite pronoun all is usually treated as plural when it refers to people, as in All were happy, All were present. In the given sentence, the intended meaning is that every person in the group felt surprised. Therefore the verb must be were surprised. The phrase was surprising would describe something as causing surprise, not the people feeling surprise, and it also mismatches the plural subject. The approach is to identify the correct combination of plural auxiliary and past participle that matches the sense of the sentence.

Step-by-Step Solution:
Step 1: Identify the subject: All, referring to all the people in the group.Step 2: Recognise that All in this sense is plural, so a plural verb is required.Step 3: Understand the meaning: the group felt surprised at discovering he was not with them, so the correct predicate should be were surprised.Step 4: Test each option in the sentence. All was surprised is wrong because was is singular. All were surprised is correct and natural. All is surprised switches to present tense, which does not match the past context.Step 5: Conclude that the best replacement for was surprising is were surprised.

Verification / Alternative check:
We can compare with similar sentences: All were shocked to hear the news and All were delighted with the result. In each case, plural were is used because the group collectively experiences an emotion. Rewriting the given sentence as All were surprised to find that he was not with them matches this pattern exactly. Using was surprising would usually require a different structure, such as It was surprising to find that he was not with them, where it becomes an impersonal subject. The sentence given, however, clearly needs a personal plural subject verb agreement structure.

Why Other Options Are Wrong:
Was surprised: This uses a singular auxiliary and conflicts with the plural subject all. It might work only if the subject were a singular noun. Is surprised: This both uses the present tense and is singular; it fails on tense and agreement. No improvement: Retaining was surprising leaves the sentence grammatically awkward because All was surprising describes the group as causing surprise rather than experiencing it, and also mismatches number. Therefore, these options do not satisfy the requirements of subject verb agreement and correct meaning.

Common Pitfalls:
Students sometimes treat all as singular because in everyday speech they may hear phrases like All is well, where all is used differently. In this question, however, all clearly refers to people and functions as a plural pronoun. Another common mistake is to not distinguish between being surprised and being surprising. The former describes emotion experienced by the subject, while the latter describes the quality of something that causes that emotion. Recognising this distinction helps avoid confusion when choosing between past participle and present participle forms.

Final Answer:
The correct replacement is were surprised, so the sentence should read, All were surprised to find that he was not with them.