Confusing Words

(i) (A) Led (B) lead is one of the many precious metals used in industry.
(ii) A child would never know what (A) ails (B) ales him. Only his mother can tell.
(iii) Today, I met a very (A) respectable (B) respectful man. He was my former maths teacher!
(iv) Demand and supply is the basic (A) principal (B) principle of economics.

Directions: In the question, there are four sentences. Each sentence has pairs of words marked as ‘A’ and ‘B’ that are italicized or highlighted. Select the most appropriate word to form grammatically and syntactically correct sentences. Then, from the options that follow, choose the correct one.