Option A is ans (plz keep patience and read below)
Isolation level:-degree to which one transaction must be isolated from resource or data modifications made by other transactions.
Lower isolation level :-- It increases the ability of many users to access data at the same time, but increases the number of concurrency effects (such as dirty reads or lost updates) users might encounter.
i.e.read uncommitted, may retrieve data that has been modified but not committed by other transactions. All of the concurrency side effects can happen in read uncommitted, but there is no read locking or versioning, so overhead is minimized.
Higher isolation level :- It reduces the types of concurrency effects that users may encounter, but requires more system resources and increases the chances that one transaction will block another.
i.e.Serializable, guarantees that a transaction will retrieve exactly the same data every time it repeats a read operation, but it does this by performing a level of locking that is likely to impact other users in multi-user systems.
Now ISO standard defines following isolation levels:-
1.Read uncommitted (the lowest level where transactions are isolated only enough to ensure that physically corrupt data is not read)
2.Read committed (Database Engine default level)
3.Repeatable read
4.Serializable (the highest level, where transactions are completely isolated from one another)
https://technet.microsoft.com/en-us/library/ms189122(v=sql.105).aspx
