You use a lens of diameter D and light of wavelength and frequency f to form

You use a lens of diameter D and light of wavelength λ and frequency f to form an image of two closely spaced and distant objects. Which of the following will increase the resolving power?

(a) Use a lens with a smaller diameter;

(b) Use light of higher frequency;

(c) Use light of longer wavelength. In each case justify your answer.