In the following questions, few sentences are given. Choose and mark the sentence which contains no grammatical or contextual error in it. If all the sentences are grammatically and contextually incorrect, mark "All are incorrect" option as the answer choice.

To find the correct sentence, let's analyze each option step-by-step for grammatical correctness and contextual accuracy:

1. "The Prime Minister, together with several cabinet ministers, are attending the summit."
Error: Subject-verb agreement. When a subject is joined with other nouns using phrases like together with, along with, or as well as, the verb must agree with the singular parent subject (The Prime Minister). Therefore, the plural verb are should be replaced by the singular verb is.
Correction: "The Prime Minister, together with several cabinet ministers, is attending the summit."

2. "It is essential that every employee submits their monthly report by the last working day."
Error: Subjunctive mood mismatch. The phrase "It is essential that..." triggers the subjunctive mood, which requires the base form of the verb (submit) instead of the third-person singular present form (submits). Additionally, there is a pronoun agreement issue since every employee is singular, making his or her grammatically preferred, though their is increasingly accepted in modern usage; however, the verb form remains a clear error.
Correction: "It is essential that every employee submit their monthly report..."

3. "He was found guilty of committing fraud and was sentenced to five years’ imprisonment."
Correct: This sentence is grammatically sound. The phrase guilty of committing fraud is correct, and five years’ imprisonment correctly uses the possessive apostrophe after the plural noun ending in 's'. The parallel structures (was found... and was sentenced...) are also correct.

4. "The new policy will effects both salaried individuals and self-employed professionals."
Error: Incorrect word usage. The modal verb will must be followed by a base verb. The word effects is a noun (or a verb meaning 'to bring about'), whereas the sentence requires the verb affect (meaning 'to influence' or 'to act upon').
Correction: "The new policy will affect both salaried individuals..."

5. "If he would have invested early, he would have accumulated a significant corpus by now."
Error: Incorrect conditional structure. In a third conditional sentence representing a hypothetical past situation, the condition clause (the 'if' clause) must use the past perfect tense (had + past participle), not would have.
Correction: "If he had invested early, he would have accumulated..."

Thus, the only grammatically correct sentence is: "He was found guilty of committing fraud and was sentenced to five years’ imprisonment."