Right choice is (b) 2k
Explanation: Given,
Let the permeability of the three layers be k1, k2, and k3.
K1=k
K2=2k
K3=3k
And z1=z2=z3=5m
For flow parallel to the bedding plane,
\(K_x=\frac{k_1 z_1+k_2 z_2+⋯.+k_n z_n}{z} \)
\(K_x=\frac{5k+10k+15k}{15} \)
∴ Kx=2k.