Step 1: Understanding the Concept.
Propulsive efficiency is really about how much of the kinetic energy added to the air ends up as useful push on the aircraft, versus how much is wasted as leftover kinetic energy in the exhaust jet trailing behind the aircraft, relative to the ground, once it leaves the engine at speed $V_e$ while the aircraft moves at $V_\infty$.
Step 2: Key Formula or Approach.
Write the useful thrust power as $P_{thrust} = \dot m (V_e - V_\infty) V_\infty$, and the total kinetic energy rate supplied to the air as $P_{KE} = \dfrac{1}{2}\dot m (V_e^2 - V_\infty^2) = \dfrac{1}{2}\dot m (V_e - V_\infty)(V_e + V_\infty)$. Propulsive efficiency is the ratio $\eta_p = P_{thrust}/P_{KE}$, which simplifies to $\eta_p = \dfrac{2V_\infty}{V_e + V_\infty}$, exactly the same result as $2/(1+V_e/V_\infty)$.
Step 3: Detailed Explanation.
For the same air-standard power, the same amount of energy pumped into the airstream per second, a turbofan spreads that energy over a much bigger mass flow $\dot m$ because of the large bypass duct around the core. Spreading the same energy over more mass means each unit of mass only needs to speed up a little, so $V_e$ ends up much closer to $V_\infty$ for the turbofan than for the turbojet, which pushes a small mass flow through a very high $V_e$ to get the same thrust. Looking at the formula in Step 2, as $V_e \to V_\infty$, $\eta_p \to 1$; so the turbofan's combination of larger $\dot m$ and a closer $V_e/V_\infty$ ratio is exactly what raises $\eta_p$. This means both option (A), more mass flow, and option (B), lower exit velocity, describe the same underlying mechanism from two angles, and both are correct reasons. Altitude, option (C), does not enter the $\eta_p$ formula at all, and the fan's power draw, option (D), is a red herring: the fan actually uses up a large fraction of shaft power to accelerate the bypass air, it does not use lesser power.
Step 4: Final Answer.
The turbofan's higher propulsive efficiency comes from its larger mass flow and correspondingly lower jet exit velocity, options (A) and (B).