Step 1: Understand the meaning of "Factum Valet".
The Latin maxim is \textit{factum valet quod fieri non debuit}, which translates to "what ought not to be done is valid when done." It is a doctrine of validation.
Step 2: Apply the doctrine to Hindu Law.
In the context of Hindu Law, this doctrine is applied to situations where a certain act or ceremony (like a marriage) has been performed and completed, but there was a minor procedural irregularity or a violation of a directory (not mandatory) rule. For example, if a marriage is solemnized without performing a non-essential ceremony, the doctrine of factum valet can be used to uphold the validity of the marriage because the core act is complete.
Step 3: Differentiate from other options.
The doctrine does not validate everything (A) or actions that violate fundamental, mandatory rules (C). Its purpose is specifically to cure minor procedural defects in a completed act, not to promote strict adherence to formalities (D), but rather to relax them in favor of the completed fact.