Correct option (B) no changes in v and halving of Z.
Explanation:
For a parallel plate transmission line shown in figure capacitance is given by,
C = εw/d
For a parallel plate transmission line shown in figure inductance is given by,
L = μ0d/w
Characteristic impedance is given by,
z = √(L/C)
z = √(μd/w x d/εw) = d/w√(μ/ε)
Hence, by reducing the spacing between the plates d by a factor of two, results in halving of Z.
Speed of propagation is given by,
v = 1/√(LC) = 1/√(μ0d/w x εw/d)
v = 1/√(μ0ε)
Hence, speed of propagation is independent from spacing between the plates.