d
The time of reverberation is defined as the time during which the intensity of sound in an autitorium becomes one millionth of initial initensity. Sabine's formula for reverberation time is

\(T=\frac{0.16 V }{\sum a s}\)

Where \(V\) is volume of hall in \(m^3\)

\(\sum a s=a_1 s_1+a_2 s_2+\ldots \ldots=\) Total absorption of the hall (room)

Here \(s_1, s_2, s_3 \ldots \ldots \ldots\) are surface area of the absorbers and \(a_1, a_2, a_3 \ldots \ldots \ldots\) are their respective absorbption coefficients

\(\frac{T^{\prime}}{T}=\frac{V^{\prime}}{s^{\prime}} \times \frac{s}{V}=\frac{(2)^3}{(2)^2}=\frac{8}{4}=2\)

Hence, \(T^{\prime}=2 T=2 \times 1=2\; s\).