$\begin {array} {1 1} (a)\;B = 2.1 \times 10^{-5} \cos \bigg[2\pi \large\frac{(x-ct)}{\lambda} \bigg]k & \quad (b)\;B = 1.4 \times 10^{-7} \cos \bigg[4 \pi \large\frac{(x-ct)}{\lambda} \bigg]i \\ (c)\;B = 1.4 \times 10^{-5} \cos \bigg[4 \pi \large\frac{(x-ct)}{\lambda} \bigg]j & \quad (d)\;B = 2.1 \times 10^{-7} \cos \bigg[2 \pi \large\frac{(x-ct)}{\lambda} \bigg]i \end {array}$