Question

They usually travel ___ their own car (in/on/by) ​

Answer 1

They usually travel by their own car

Explanation:

Preposition-

It is used as a link between between nouns, pronouns to object or other part of the sentence.

For example:

We have a sentence

• She looks beautiful in red dress.
• The tiger sprang upon the deer.

Now in above given examples by and upon are used as a link between the subject and the object.

As asked above in the question: That is why "by" is used as a preposition.

"By" is used as a preposition for the means of transport.When we travel by some means of transport we use "by".