The correct and easiest option is B

SELECT Name , ( SELECT Id , Title FROM Contacts ) FROM Account

Doesn't make sense only to add LIMIT clause in order to keep safe gorvernor limits.
Code is just doing querys.

For example, changing the code and removing 'for' loop:

List <Account> aList = [SELECT Id FROM Account];
Set<Id> aIdSet = (new Map<Id, sObject>(aList)).keySet();
List<Contact> cList = [SELECT Id FROM Contact WHERE AccountId IN aldSet];

It's not necessary to think about the governor Limits.
So if you remove the loop, you will improve the code and keep safe about governor Limits.

So my answer will be A.