Amonopolistically competitive industry combines elements of both competition and monopoly. it is correct to say that the competitive element results from:

a) a relatively large number of firms and the monopolistic element from product differentiation.

b) product differentiation and the monopolistic element from high entry barriers.

c) a perfectly elastic demand curve and the monopolistic element from low entry barriers.

d) a highly inelastic demand curve and the monopolistic element from advertising and product promotion.

A) a relatively large number of firms and the monopolistic element from product differentiation.

Explanation:

A monopolistically competitive industry has the elements of monopoly as product differentiation. Since the products produced in are different in some way and thus may offer differing utilities. This allows the firms in the industry to vary their supply to influence prices as this differentiated product is only produced by them. This is reminiscent of a monopoly.

However, at the same time - there may be substitutes with slight variations as  there are a relatively larger number of companies producing differing products. This offers as an option to customers and helps the market act as competitive.

Option B only focuses on the monopolistic elements. Option C is fundamentally wrong as low entry barriers is not a monopolistic element. Option D gives us a monopolistic element of advertising that can act as differentiation but a highly inelastic demand curve goes against the perfect competition - this nullifies the argument.

Hope that helps.