The selection of Spiral Model based on characteristics of requirements:
(A) Are requirements easily understandable and defined?
(B) Do we change requirements quite often?
(C) Can we define requirements early in the cycle?
(D) Requirements are indicating a complex to be built

Explanation

The Spiral Model is an evolutionary, risk-driven model, so it is chosen exactly when requirements cannot be frozen at the start. It suits projects where requirements change quite often (B), because each spiral re-visits requirements, and where the system to be built is large and complex (D), because every loop performs explicit risk analysis and prototyping. Conditions (A) and (C) — requirements that are easily understood and can be defined early — describe the ideal setting for the Waterfall model, not the Spiral. Hence (B) and (D) only is correct.