Choose the idiom/phrase that can substitute the highlighted group of words meaningfully.
Stop preaching. You must understand that it is tougher to do something in reality than to say or think.

Correct option is A

Option (a) is the correct idiom: "Easier said than done."
Meaning: Something that is easy to say but difficult to actually do.
Example: "Starting your own business is easier said than done."
Other options:
Bolt from the blue: A sudden and unexpected event.
Make no bones about: To speak plainly and directly.
Chew the cud: To think something over.