A sentinel value and signals that there are no more values to be entered:
a. is a different data type than the values being processed
b. is a special value that cannot be mistaken as a member of the list
c. indicates the start of a list
d. guards the list

The answer is "Option b".

Explanation:

Sentinel [..] is a special value, which existence ensures the end of a loop, which performs structured data. It allows the information end to be identified where no other means are available for this, and wrong choices can be described as follows:

ιт ιѕ ωнєи ѕσмєσиє ρυтѕ ιи α ¢σ∂є σя ѕσмєтнιиg тнαт вяιиgѕ α νιяυѕ, σя мєѕѕєѕ уσυя gαмє υρ, σя αℓℓσωѕ тнєм тσ ¢нєαт.

αи∂ уєѕ ι нανє вєєи нα¢кє∂ вєfσяє(ιт αιит fυи)

нσρє ι ¢συℓ∂ нєℓρ уσυ.