Which word best describes a situation that is not securely established?

The word "precarious" effectively conveys a sense of instability or insecurity in a situation. It implies that something is uncertain, vulnerable, or dependent on chance, making it suitable for describing circumstances that lack a firm or secure foundation. In a precarious situation, there may be a risk of failure or collapse, highlighting the lack of assurance regarding safety or stability.

The other terms do not share this same context of insecurity. "Unremitting" refers to something that is persistent or relentless, often without pause or let-up, which does not imply a lack of security. "Tender" generally relates to something being gentle or delicate, which does not address the concept of establishment or stability. "Terse" describes a brief and to-the-point manner of speaking or writing, which has no direct connection to the idea of being securely established or not. Therefore, "precarious" is the most fitting choice to describe a situation that is not securely established.