Topic: Idioms & Phrases,

Best Answer Explained in Below-:

The phrase "all at once," which translates to "হটাৎ বা আকস্মিকভাবে" in Bengali, denotes an occurrence that takes place unexpectedly and abruptly, without any warning or gradual transition. Among the provided options, "Suddenly" (Option A) is the only choice that accurately captures this concept.

When something happens suddenly, it suggests an immediate and unanticipated change in circumstances or events, aligning perfectly with the notion of happening "all at once." Conversely, "Variably" (Option B) refers to something that changes or fluctuates over time, which contrasts with the idea of a sudden event. "Friendly" (Option C) describes a demeanor or attitude, which has no relevance to the concept of abruptness or immediacy. Lastly, "None" (Option D) does not provide an alternative that conveys the intended meaning.

Therefore, "Suddenly" stands out as the correct answer due to its synonymous relationship with the phrase “all at once,” both indicating an event that occurs unexpectedly and without prelude.