Among the following, an example of rectilinear motion is

A train moving on a straight bridge 

Rectilinear motion refers to the movement of an object along a straight path. This type of motion is characterized by the object maintaining its trajectory without deviating to curves or circular paths.

For example:

• A train moving on a straight bridge: This is the correct answer because the train moves along a linear path without any change in its direction.
• Swaying of a tree: This represents oscillatory motion, not rectilinear motion.
• A child on a merry-go-round: This describes circular motion, which involves rotation along a circular path.
• The motion of a pendulum in a wall clock: This is an example of periodic motion, as it repeats at regular intervals.

Therefore, an example of rectilinear motion is clearly observed in a train moving on a straight bridge. This straightforward movement is the hallmark of rectilinear motion, differentiating it from other types of motion like oscillatory, periodic, or circular.