The word "vulnerable" means susceptible or easily harmed or affected by something. In the given sentence, it is mentioned that a human being is always vulnerable to other human beings. Here, vulnerable implies that a human being is susceptible to harm or can be affected negatively by other human beings.

Let`s now dissect each option to better understand their meanings and determine the closest synonym of "vulnerable".

Option 1: Resilient - This word means that a person is able to recover quickly from difficulties or challenges. It is not the closest synonym for "vulnerable" as it implies strength and the ability to bounce back.

Option 2: Elastic - This word refers to a material or substance that is able to stretch and return to its original shape. It does not convey the meaning of being easily harmed or affected.

Option 3: Defenceless - This option closely matches the meaning of "vulnerable." It means lacking protection or being unable to defend oneself. In the context of the sentence, it accurately describes a human being`s susceptibility to harm from other human beings.

Option 4: Crude - This word means lacking refinement or manners and does not align with the meaning of "vulnerable."

Therefore, the correct answer is option 3 - "defenceless