The correct answer is option 3, which states that the main reason behind the boycott of the recent general elections in Bangladesh by the main opposition parties was the demand for holding the elections under a neutral government, which was not fulfilled.

Option 1 suggests that the boycott was due to the demand for a fresh electoral roll not being fulfilled. This means that the opposition parties wanted a new list of registered voters, but there is no information to indicate that this was the main reason behind the boycott.

Option 2 states that the demand for allowing international observers for the poll was not fulfilled. While international observers are often present during elections to ensure transparency and fairness, it is not clear from the question whether this demand played a major role in the boycott.

Option 4 implies that the demand for stopping the prosecution of those involved in 1971 war crimes was not fulfilled. While this may be a valid concern for the opposition parties, it is not mentioned as the main reason behind the boycott in the question.

Therefore, option 3 is the most accurate answer.