Question

what is causing this wave?
water molecules in
the tank are
displaced by the
falling drops.
the water drops
travel from left to
right across the
tank.
simulations used in this interactive are by phet interactive simulations
university of colorado boulder, licensed under cc - by - 4.0 (https://phet.colorado.edu).

Explanation:

Step1: Analyze wave - formation concept

In wave formation, when an external force (like falling drops) acts on a medium (water in the tank), it displaces the particles of the medium.

Step2: Evaluate each option

• For the option "Water molecules in the tank are displaced by the falling drops": This is in line with the basic concept of wave - generation where a disturbance (falling drops) displaces the medium (water molecules) to create a wave.
• For the option "The water drops travel from left to right across the tank": In a water - wave (a transverse wave in this case, assuming a simple tank - wave scenario), the water molecules oscillate up and down (not travel horizontally in the direction of wave - propagation). The wave energy travels, not the individual water drops in the sense described in this option.

Answer:

Water molecules in the tank are displaced by the falling drops.