Explanation

The plastic limit is the moisture content below which a cohesive soil ceases to be plastic and begins to crumble. While 'sticky limit' is sometimes used in older texts to describe the point where soil stops sticking to tools, the standard geotechnical term for the boundary between plastic and semi-solid states (where stickiness significantly reduces) is the plastic limit. However, strictly speaking, the 'sticky limit' is defined as the moisture content below which soil no longer sticks to metal. Given the specific phrasing 'does stick to metal tools', 'Sticky limit' is technically the precise definition for that phenomenon, even though it is less commonly tested than the Plastic Limit. Wait, let me double-check standard definitions. The sticky limit is indeed defined as the water content below which soil no longer adheres to metal. The plastic limit is where it stops being plastic. In many MCQs, 'Sticky limit' is the intended answer for stickiness. However, some sources equate the sticky limit with the shrinkage limit or consider it a subset of the plastic limit range. Let's look at common exam patterns. Often, 'Plastic limit' is the answer for 'crumbling'. 'Shrinkage limit' is for volume change. 'Sticky limit' is specifically for stickiness. Therefore, C is likely correct based on the specific wording. But wait, is 'Sticky limit' a standard term in modern geotechnical engineering? It is less common than Atterberg limits (LL, PL, SL). Some texts say the sticky limit is approximately equal to the plastic limit. If the question asks for the standard Atterberg limit, it might be B. But the definition 'does stick to metal tools' matches 'Sticky limit' exactly. I will stick with C being correct because it matches the specific definition provided in the question.