What is meaning of current rating 100/5 printed on CT?? /

Most likely 100 Amps in 5 amps out

AKA Ratio

Current transformer - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Current_transformer)

This must be one heck of a test your taking.

Did you sleep through all the classes?

It simply reduces the actaul current in the conductor being measured by the ratio as noted on your CT to a more manageable level.
i.e. if your conductor is running at say 80A, then the CT will be 100:5 (20:1) ratio, which is more manageable for metering or measurement purposes. The output side shall be 4A

In simple language this is current transformation ratio called as ct ratio.