A safe expression in relational calculus is one that is guaranteed to yield:

Correct option is A

In relational calculus, a safe expression is an expression whose result is guaranteed to be finite and whose values are restricted to the active domain of the database.
The main purpose of the safety condition is to ensure that the query produces a finite relation, rather than an potentially infinite set of tuples.
For example, consider a database with a finite active domain. A safe relational calculus expression ensures that the query result is limited to tuples constructed from values occurring in the database (or otherwise appropriately bounded by the query).
Therefore, a safe expression is guaranteed to yield a Finite number of tuples.
Information Booster
1. Relational calculus is a non-procedural query language; it specifies what data is required rather than how to retrieve it.
2. A safe expression guarantees that the result of the query is finite.
3. Safety also restricts the values appearing in the result to the active domain of the database.
4. Both Tuple Relational Calculus (TRC) and Domain Relational Calculus (DRC) require safety considerations.
5. Without safety restrictions, a relational-calculus query could theoretically produce an infinite result even when the database itself is finite.
Additional Knowledge
· Tuple Relational Calculus (TRC): Variables represent tuples.
· Domain Relational Calculus (DRC): Variables represent individual attribute values.
· Safe query: Produces a finite, well-defined result based on the database's active domain.