A protective cap or hook-like covering on the developing capsule in moss is known as __________.
A. Spine
B. Calyptra
C. Lodicule
D. Calyptrogen

Hint: Moss is a member of the division Bryophyte of the kingdom Plantae. Bryophytes are known as the non-vascular amphibians of the plant kingdom as they are terrestrial but they require water for the transfer of their gametes. Moreover, the vascular tissues are absent in these plants.

Complete answer:
Mosses are a member of the class Bryopsida of Bryophytes. The Bryopsida is considered the most advanced group of bryophytes. They are commonly known as the leafy bryophytes. Their gametophyte passes through two phases that are the temporary protonemal phase and permanent leafy gametophore.
The sporophyte of mosses consists of three parts. These three parts are the capsule, seta, and foot. The process of spore formation occurs inside the capsule of the spore sac. The capsule has an outer protective covering resembling a cap which is known as the operculum or calyptra. On maturation, the capsule bends, and the length of seta increases. After this, the operculum disintegrates in moisture and the peristomial teeth open and the spores are released.
Thus, based on the above information we can conclude that a protective cap or hook-like covering on the developing capsule in moss is known as Calyptra.

Hence, the correct answer is an option (B).

Note : The mosses help in the formation of the soil as they are the initial members to grow on the rocks. Sphagnum also has good water-absorbing ability due to which it is used as an antiseptic. It is also known as peat moss as it forms coal-like peat.