Eclecticism in architecture refers to a design approach or style that draws inspiration, ideas, and forms from a diverse range of historical architectural styles and sources, often combining them within a single work. Instead of strictly adhering to one historical style or a new, singular design philosophy, eclectic architects select and combine elements from different periods and cultures.
Let's evaluate the options:
(a) Rigidity in adhering to a single historical style: This describes revivalist styles (e.g., Neoclassicism, Gothic Revival) where architects try to faithfully replicate or reinterpret a specific historical style. Eclecticism is the opposite; it embraces multiple styles.
(b) Incorporation of elements from various architectural styles: This is the defining characteristic of eclecticism. Architects consciously choose and combine motifs, forms, and details from different historical precedents (e.g., Classical, Gothic, Renaissance, Baroque,
as well as non-Western styles) to create a new, composite design.
(c) Emphasis on minimalist design and functionality: This describes Modernist principles (e.g., "form follows function," rejection of historical ornament, simplicity). Eclecticism often involves ornamentation and forms drawn from historical styles, which can be complex rather than minimalist.
(d) Focus on the use of innovative materials and technologies: While eclectic architects might use modern materials and techniques, their defining characteristic is the stylistic borrowing, not necessarily a primary focus on material/technological innovation (which is more typical of Modernism or High-Tech architecture).
Therefore, the characteristic that defines eclecticism in architecture is the incorporation of elements from various architectural styles. Eclecticism was particularly prominent in the 19th and early 20th centuries.
\[ \boxed{\text{Incorporation of elements from various architectural styles}} \]