The word, "if" is a subordinating conjunction which connects a main (independent clause) and a subordinate (dependent) clause. The main clause expresses a condition, upon the happening of which, another action/event (described by the dependent clause) will or might happen. Here the main clause is: "If it snows this weekend". As the main clause is in the future tense, the dependent clause will also be in future tense only.
Option A "The verb, "go" is in simple present tense. Hence, use of the word in the blank is grammatically incorrect.
Option B: The word, "shall" is used along with the first person pronoun (I/We) to express an action/strong assertion/intention in the future tense. The clause, "we shall go skiing" expresses an intention of an action in the future, which is conditional upon the main clause. Hence, this option is correct.
Options C is incorrect as the phrase, "could going" is grammatically incorrect and conveys no meaning.
Option D is incorrect as the phrase "are going" is in present continuous tense. It cannot describe an action in the future tense. Hence, this option is incorrect.