How to enable Oracle Database In-Memory feature in Oracle 11g R2 to reduce report generation time?
a) + b) The In-Memory Column Store option was introduced in version 12.1.0.2, it is not available in 11g.

Changes in Oracle Database 12c Release 1 (12.1.0.2)

You can cache tables in earlier versions as well, but the new In-Memory option is different. The point of the new In-Memory option is it stores tables in memory in columnar format, instead of the default row level format.

c) Logical standy, Goldengate (requires extra licence) or Streams.