Explanation

The preposition 'on' indicates that something is already in the position. 

Explanation

For is used for period.

Since is used for point of time.

He has eaten nothing for a long time means اس نے کافی عرصے سے کچھ نہیں کھایا

Explanation

The new and relatively short channel formed when a stream cuts through the neck of an oxbow

Explanation

Their share price fell to below ten rupees yesterday.

Explanation

"She is worried about her exams". 

When expressing concern or anxiety about something, the correct preposition to use with "worried" is "about". 

Explanation

He applied for the job. 

اس نے نوکری کے لیے اپلائی کیا۔

The verb "apply" is commonly followed by the preposition "for" when referring to a request for something

Explanation

Generally oppose is followed by To.

Oppose means مخالفت کرنا

We opposite to policy of yours. 

Explanation

The supervisor thought on the most suitable person to do the job 

Explanation

He scoffed at the idea of revolution

Here, at denotes the position in which he scoffed.

So, "at" is right here.

Explanation

From = point of time (present, past & future)

He waited for me from 7 MP to 8 PM means اس نے شام سات بجے سے آٹھ بجے تک میرا انتظار کیا۔

Note: He waited for me from 7 PM till 8 PM is also the correct sentence.