SQL Server Installation choices I'm installing SQL Server 2008 R2 which would be helpful in 

A. querying database.

B. Analysis Services

C. Reporting services and BIDS (SSRS)

D. Powerpivot in Sharepoint

During installation I get two options:

a. SQL Server feature installation: this would install database engine, analysis services, reporting services etc

b.  SQL Server for powerpivot for sharepoint: This would be helpful for powerpiviot in Sharepoint. Not sure this option would install all the options mentioned in first option - SQL Server feature installation.

My Question: Which option I should select? If i go for option-a at present, can I later change it to powerpivot option i.e option b.???
Install the tools you need now, and no others.  You can always add more features to the installation later.

If you're not installing reports or PowerPivot into your existing SharePoint installation, don't install them.

When most people say that they want to get "SQL Server" installed, they mean just the database engine.  If that sounds like you, install the Database Engine and Management Tools, and you'll have a basic SQL Server up and running.