The correct answer is D. Spongiosis. 

Recognize the pruritic palmar monomorphic vesicles as most consistent with
dyshidrotic eczema (i.e., pompholyx). Spongiosis is a key feature to separate it
from other diagnoses.

Eosinophilia may be present in contact dermatitis or bullous pemphigoid, a
floor-pattern salt-split skin may be present in epidermolysis bullosa acquisita,
leukocytoclasia may be present in vasculitis, and parakeratosis may be present in psoriasis.

References:

Masuda-Kuroki K, Murakami M, Kishibe M, et al. Diagnostic histopathological
features distinguishing palmoplantar pustulosis from pompholyx. J Dermatol.
2019;46(5):399-408.