Solution

The correct answer is (B), (C) and (D) Only.

EXPLANATION:

• (A) NRZ is a bipolar scheme in which the positive voltage defines bit is 0 (zero).
  • This is incorrect. Non-return-to-zero (NRZ) is not a bipolar scheme. Also, in bipolar schemes, typically, the zero bit is represented by no voltage, not a positive one.
• (B) NRZ-L and NRZ-I both have an average signal rate of N/2.
  • This is correct. NRZ-level and NRZ-invert are binary coding schemes where the signal drops (but does not return to zero) in the middle of the bit period. The average signal rate in both cases is N/2.
• (C) The idea of RZ and NRZ-L are combined into the Manchester scheme.
  • This is correct. Manchester coding, used in data communication, combines elements of RZ (Return to Zero) and NRZ-L (Non-Return to Zero Level) to encode data.
• (D) NRZ-L and NRZ-I both have DC component problems. 
  • This is correct. Both NRZ-L and NRZ-I maintain a constant voltage level for the entire bit period, which can create a direct current (DC) component when long sequences of identical bits are sent.
• (E) The minimum bandwidth of Manchester and Differential Manchester is 3 times that of NRZ.  
  • This is incorrect. The minimum bandwidth for Manchester and Differential Manchester schemes is actually twice that of NRZ, not three times.

Hence, looking at every given statement, the correct ones are B, C and D.