(a) 1. Progressive taxes help in reducing the income inequality among different income groups of people in a society. The rich pays a higher average tax rate than the poor. So the gap between the rich and poor is narrowed down. But in case of proportional taxes both rich and poorer persons are to pay the tax at the same rate. 

2. Progressive tax is more elastic than proportional tax.

(b) Under monopolistic competition there are many sellers in the market which sells differentiated products. Whereas in monopoly there is a single seller selling homogeneous or non-homogeneous product.

(c) Private sector helps in rapid economic growth. It is more productive than public sector and has more competitive ability and hence earns more profit in less time.

(d) A direct tax is an equitable, economical, certain, elastic and productive and also increases the civic sense. Whereas indirect tax is unequitable, uneconomic, uncertain, and does not create civic sense.

(e) A credit card gives the facility to the cardholder to purchase goods and services from various shops without paying the currency notes (cash) at that time. The card issuing banks pay the seller immediately after the transaction and charge the buyer after one month or so, as the condition specified. Moreover, risk of carrying currency notes is also overcome by a credit card.