Choose the most appropriate option to fill in the blank.____ the gove…

The correct answer is Option (2) → Unless

Conjunction of condition:

• Unless means "if ... not"; it introduces the one condition without which the result in the main clause will not happen.
• This sentence: the problem will be solved only if the government takes stringent (strict) measures; "Unless the government takes ..." = "If the government does not take ...", and then "the problem will not be solved".
• Complete sentence: Unless the government takes stringent measures, the problem will not be solved.
  Example: Unless you hurry, you will miss the bus.

Why the Other Options are Incorrect?

Option 1: Although — Incorrect. Although shows contrast; it would mean that even strict measures will not solve the problem, which is not the sense of the sentence.
Example: Although it was raining, they played the match.

Option 3: As — Incorrect. As shows reason (because); it would make the strict measures the cause of the problem not being solved, which makes no sense.
Example: As he was ill, he stayed at home.

Option 4: When — Incorrect. When shows time; it would say that the problem stays unsolved at the time the government acts, the reverse of the sense.
Example: Call me when you reach home.