The correct option is B As soon as
Conjunctions are words that link other words, phrases, or clauses together. Subordinating Conjunctions help to link or connect a dependent clause to an independent one. The commonly seen relationship between the two clauses are cause- and- effect or contrast but it can be any other type as well. When, after, before, until, since, while, once, as and as soon as are subordinating conjunctions which can be used to connect an action or an event to a point in time.As soon as comes at the beginning of a subordinate clause. We use as soon as to show that something happens immediately, i.e. ‘at the very moment another action is completed’, or ‘shortly after another action is completed’.In the given sentence it is inferred that the moment the storm abated immediately people began to walk out of their houses. Thus Option B is correct. We use as long as to refer to the intended duration of a plan or idea, most commonly referring to the future. We always use the present simple to refer to the future after as long as. Thus Option A can be ruled out as it refers to the moment and not to the intended duration of a plan. Option C is incorrect as provided (that) is used to say what must happen or be done to make it possible for something else to happen. As a conjunction, since is used to connect two clauses. Clauses introduced by since typically show one of the two associations: time or cause/reason.Thus the option can be ruled out.