IIBA AAC examination is multiple-choice (MCQ) type test comprising of 85 questions. The duration of the exam is 2 hrs. To prepare for the exam, you need to go through the Agile extension v2 guide which comprises of three horizons and 24 techniques. Once you go through the guide, you need to start taking tests and other drills to check if you have understood the concepts and can apply them. All the questions are scenario based so applying the concepts correctly is important. Practicing with chapter-based questions and full-length simulations is important.

In the agile world, business analyst role is not documented but that mean that agile approaches do not need Business analysts. For example, in Scrum, there are three roles – Scrum Master, Product Owner and Scrum Team.
The product owner is pretty much a Business analyst function. Product Owners are responsible for eliciting, managing, and prioritizing the requirements. Business analysts are also part of the Scrum team where they work with the product owners for capturing requirements and work with the developers to get the requirements implemented.

IIBA Canada and iSQI Germany offer Agile Analysis Certification (AAC) and Certified Agile Business Analyst (CABA) Certifications. Scrum alliance and Scrum.Org offer Product Owner certifications.
The decision to choose a certification depends on eligibility criteria (whether you meet that or not), your specific requirements and the reputation of the certifying body.

IIBA does not have any eligibility criteria for AAC Certification. This means anybody can go for it.
However, IIBA recommended having couple of years of experience in the software development and agile domain.

Business analysis is a practice which is applicable in all the approaches – whether waterfall or agile.

Business analysis practice comprises of skills like requirements elicitation, requirements analysis and modelling, preparation of specifications, verification, and validation. These skills are generic and applies to all the type of projects.
Though, more and more new projects are using agile approaches so learning the finer points of Agile approaches and how business analysis practice is applicable in agile, is important and needed.