B

Step 1: Calculate the total emissions in the given time intervals. During the first 2 seconds,
N1 = n\beta
In the next 2 seconds, N2 = 0.75n\beta.
Step 2: Total emissions in 4 seconds = N1 + N2 = n\beta + 0.75n\beta = 1.75n\beta.
Step 3: The mean-life \tau (mean life of a radioactive material) is given by the relation \tau = \frac{T}{N}, where T is the total time elapsed and N is the total emissions.
Step 4: Thus, Mean-life \tau = \frac{4 seconds}{1.75} \approx 2.29 seconds.
Rounding to the nearest whole number: 2 seconds. Therefore, the answer is B.