Fill in the blank with the most suitable word from the options given below:

Most dogs snarl ________ you pull their tail.

The correct option is B if
The sentence calls for a conditional conjunction because we have to show how the first clause of the sentence is dependent on the second clause or simply put, to explain why the dogs' snarl. So option B is the correct answer. If is used to express general truths or facts. It is important to note that both parts of the sentence are in simple present tense.

Option A is incorrect because unless is used in sentences to mean the same as if..not. Using this option would rid the sentence of its meaning.

Even though both options C and D are subordinating conjunctions which can be used to indicate a reason, either of them doesn't seem to be an apt answer as it sounds grammatically incorrect.