Which word refers to something that is shielding or protective?

The term that most directly relates to something that is shielding or protective is "defensive." This word conveys the idea of taking action or forming a barrier to protect against threats or harm, embracing the proactive nature of shielding. In various contexts, "defensive" can describe strategies, attitudes, or physical structures that serve to safeguard oneself or others from potential dangers.

While the other options also convey protection in some way, they do not capture the same comprehensive approach as "defensive." For example, "shielding" is focused specifically on the act of providing a shield, which can be interpreted as a more passive form of protection. "Guarding" implies watchfulness and active protection, yet it does not encompass the broader concept of anticipating and preventing threats. "Fortified" suggests something that has been strengthened or reinforced, but it often relates to physical structures rather than the more generalized action or attitude of protection that "defensive" embodies. Hence, "defensive" stands out as the best choice for describing something that is shielding or protective.