Step 1: Couple protection rate measures what fraction of eligible couples are using an approved contraceptive method, and it is a marker of family planning uptake.
Step 2: An NRR of 1 is the demographic target where each generation merely replaces itself, leading to eventual population stabilisation.
Step 3: Demographic experience shows this replacement-level fertility is attainable only when more than 60 percent of eligible couples are effectively protected.
Step 4: So among the options, the threshold couple protection rate associated with NRR equal to 1 is 60 percent.
\[\boxed{60\%}\]