According to Graham's law of diffusion of gases,the rate of diffusion of a gas is proportional to the square root of the density.

From the question, the given parameters are:

Volume of Oxygen, V\(_o\) = 30cm\(^3\), Time of diffusion of Oxygen, t\(_o\) = 7s, Vapour density of Oxygen, d\(_o\) = 16

Volume of Chlorine, V\(_c\) = 60cm\(^3\), Time of diffusion of chlorine, t\(_c\) = ?,  Vapour density of Chlorine, d\(_c\) = 36

\(\frac{t_o}{t_c}\) = \(\sqrt{\frac{d_o}{d_c}}\)

\(\frac{7}{t_c}\) = \(\sqrt{\frac{16}{36}}\)

\(\frac{7}{t_c}\) = \(\frac{4}{6}\)

t\(_c\) = \(\frac{7}{4}\times {6}\)

t\(_c\) = \(\frac{21}{2}\)

Recall that the volume of Chlorine is twice that of Oxygen. Hence, t\(_c\) = 2t\(_o\)

t\(_c = 2\times \frac{21}{2}\) 

t\(_c\) = 21sec - Option C