Select the word or phrase that best completes the following sentence:…

The correct answer is Option (4) → actually, but

Conjunction of contrast:

• But joins two ideas that contrast with each other.
• Actually (in fact, really) adds emphasis to the first part.
• This sentence: the flowers stay invisible for years, but then appear suddenly after rain. The two parts contrast, and of the four choices only this one puts a contrast word (but) in the joining position.
• Complete sentence: Desert flowers can be invisible for years actually, but appear suddenly after heavy rain.

Why the Other Options are Incorrect?

Option 1: alternatively or — Incorrect. "Alternatively" and "or" offer a choice between two things. The flowers do not either stay invisible or appear, they do one and then the other.
Example: You can go by train or, alternatively, take a bus.

Option 2: consequently and — Incorrect. "Consequently" shows a result. Appearing after rain is not a result of being invisible for years.
Example: He missed the bus and consequently reached school late.

Option 3: nevertheless, so — Incorrect.

• "Nevertheless" shows contrast, but "so" shows a result.
• The two together give a confused meaning.
  Example: It was raining; nevertheless, the match went on.

Contrast: "but" and "nevertheless" show a contrast, "or" and "alternatively" offer a choice, and "so" and "consequently" show a result.