Pull someone’s leg means to form fun or play a joke on someone. This idiom was first employed in the 1800s in England when street robbers wont to ‘pull someone’s leg’ employing a wire to trip the person and rob their valuables.
Example- i used to be pulling his leg after I told him that dogs can talk. Here, we are able to understand that ‘X’ played a joke on ‘Y’.
Let’s examine the given options to pick the right option:
To tease somebody- It means to create fun of somebody. it's the identical meaning because the above-given idiom. Hence, it's the proper option.
To throw somebody out- It means to expel or force someone out of an area or an activity. it's a special meaning from the above-given idiom. Hence, it's an incorrect option.
To trip someone purposely- It means to intentionally catch someone’s foot and make them fall or stumble. It expresses the literal meaning of the above-given idiom. Hence, it's an incorrect option.
To stretch someone’s leg- It means to loosen one’s leg muscles after sitting down for a protracted time. Hence, it's an incorrect option.
The correct answer is Option ‘a’.