Take the five statements one at a time.

A — "Reflection coefficient is a change in magnitude only, with constant phase." FALSE. The reflection coefficient is a complex quantity,

\(\Gamma=\dfrac{Z_L-Z_0}{Z_L+Z_0}=|\Gamma|e^{j\theta}\)

so a reflection generally changes both magnitude and phase. Only in special cases does the phase stay fixed — a short circuit gives θ = 180°, an open circuit θ = 0° — and a reactive load produces any angle in between.

B — "Transmission coefficient changes both magnitude and phase of the transmitted wave." TRUE. Likewise complex, and related to Γ by

\(\tau = 1+\Gamma = \dfrac{2Z_L}{Z_L+Z_0}\)

C — "On the Smith chart, λ/2 is equivalent to 2π." TRUE. Moving a distance d along a line rotates the reflection coefficient by \(2\beta d\); with \(\beta = 2\pi/\lambda\), a movement of λ/2 gives a rotation of

\(2\times\dfrac{2\pi}{\lambda}\times\dfrac{\lambda}{2}=2\pi\)

i.e. one complete revolution of the chart. This is why the impedance on a lossless line repeats every half wavelength, and why the outer scale of the Smith chart is calibrated 0 to 0.5 λ.

D — "For a distortionless line, RL = GC." FALSE. Heaviside's condition is

\(\dfrac{R}{L}=\dfrac{G}{C} \quad \Longleftrightarrow \quad RC = LG\)

Under it the attenuation \(\alpha=\sqrt{RG}\) is frequency-independent and the phase velocity is constant, so all frequency components arrive together and the pulse shape is preserved. The statement pairs the wrong quantities.

E — "Directivity of an antenna can be less than 1." FALSE. Directivity is the ratio of the maximum radiation intensity to the average over all directions:

\(D=\dfrac{U_{max}}{U_{avg}}\ \ge 1\)

Since a maximum can never be below the average, D ≥ 1, with equality only for the (hypothetical) isotropic radiator. Note that gain — which includes efficiency, \(G=\eta D\) — can be less than 1 for a lossy antenna, which is the confusion this statement plays on.

Hence, the correct answer is B and C only.