Question

They are drowned ……………………… their own thoughts. The correct preposition is ?

Answer 1

In (inside of something)

Explanation:

They were drowned/are drowning (make sure the subject agrees with the verb; are is in the present tense, drowned is in the past tense) in their own thoughts.

They are not on their thoughts, they are being drowned by their thoughts (such as being drowned by water, you would be in the water, not on it).