Explanation

I am indebted to you for your help

میں آپ کی مدد کے لیے آپ کا مقروض ہوں۔

In the sentence the word "to" is used as a preposition to indicate the recipient of the indebtedness.

Explanation

The preposition "for" is used in the sentence "Cantonments are requisite for happiness" to indicate the purpose or necessity of cantonments in relation to happiness.

It suggests that cantonments are essential or necessary for the attainment or maintenance of happiness.

Explanation

We use on to refer a surface.

On is a preposition here.

Feed is generally followed by on.

Cows feed on grass گائے گھاس کھاتی ہیں

Explanation

He is the youngest among the seven children.

Between دو لوگوں کیلیئے استعمال ہوتا ہے

Among دو سے زائد لوگوں کیلیئے استعمال ہوتا ہے

Explanation

We generally use of after guilty.

Of is a preposition here.

Guilty of murder means قتل کا مجرم

Explanation

Deal with a matter means کسی معاملے کو نمٹانا

Deal in something means کاروبار کرنا یا روزی روٹی کمانا

Deal out تقسیم کرنا

It is difficult to deal with a child. 

Explanation

Stumble means ٹھوکر کھانا

Stone means پتھر

Ditch means کھائی 

Fell into something means کہیں گر پڑنا

We use into to refer a motion.

Into is a preposition here.

Explanation

Blind means اندھا

Blind to means عقل سے اندھا

Blind in means ایک آنکھ سے اندھا

Explanation

Over is sometimes also used to cover the whole surface no matter whether there is physical contact or not.

The bridge was built over the river means دریا پر پل تعمیر کی گئی 

Explanation

'At' indicates specific time, location or position.

Hence it is the correct preposition to be used in the sentence.