Which of the following words is closest in meaning to “Sheath”?

Sheath means a protective covering.

• Correct answer - protective covering: it has the same meaning as Sheath.
• an unfolding: a gradual opening out - wrong, it does not mean the same as Sheath.
• blade: the cutting part of a knife - wrong, it does not mean the same as Sheath.
• decayed: rotten - wrong, it does not mean the same as Sheath.