Identify a standard POS expression (variables A, B, C, D): Which option is in <em>standard</em> product-of-sums (POS) form, where each sum term includes all variables (possibly complemented)?

Introduction / Context:
In standard POS form for variables A, B, C, D, each sum (maxterm) must contain all four variables, either complemented or uncomplemented. This ensures a canonical structure that directly matches maxterm indices in a truth table.

Given Data / Assumptions:

• Variable set is A, B, C, D.
• Standard POS requires every sum term to list all variables.

Concept / Approach:
Check each option: if any sum term omits one of the variables, it is not in standard POS (though it may be a valid non-standard POS). Only options where every sum term contains A, B, C, and D (with complements as needed) qualify as standard POS.

Step-by-Step Solution:

Verification / Alternative check:
Expanding the correct option to maxterms demonstrates each term maps to a specific maxterm index set.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing valid POS with standard POS; “standard” implies all variables present per sum term.

Final Answer:
(A + B + C + D)(A + B' + C + D)