Which of the following is/are possible for a modified allele ?
i) Can produce no enzyme
ii) Can produce a non functional enzyme
iii)Can produce less efficient enzyme

The phenotype/trait will only be dependent on the functioning of the unmodified allele. The unmodified allele, which represents the original phenotype is the dominant allele and the modified allele is generally  recessive allele ,the modified allele could be responsible for production of production of

(i) the normal/less efficient enzyme, or

(ii) a non-functional enzyme, or

(iii) no enzyme at all