Hi Neetin,
2. How we choose cherterstic polynomial for lfsr?
The characteristic plynomial is the LFSR connection which will give the maximum number of random values before it starts to repeat the values. Each LFSR will have a characteristic polynomial, which is pre-determined. There are equation to find that, which I don't remember just now.
Some Polynomials for Maximal LFSRs
https://en.wikipedia.org/wiki/Linear_feedback_shift_register
You can see the polynomials for the different length of the LFSRs is given in the link above.
1. How we decide for the TAP point in a CUT(circuit under test)?
I didn't understand the question... :-(
-cheers
vlsi_eda_guy