Marchantia is considered heterothallic because it is

The correct option is A Dioecious

Introduction to heterothallic organisms
Heterothallic organisms are those which have the male and female reproductive structures on separate thalli.

Heterogametic
An organism is said to be heterogametic if it produces two different types of gametes, for example, humans.

Bisexual
Bisexual organisms are those which contain both male and female reproductive organs and are capable of producing both male and female gametes.

Monoecious
Monoecious organisms are those which bear both male and female reproductive structures in the same individual.

Dioecious
Dioecious organisms are those in which the individuals bearing the male and female reproductive organs are separate.
Plant body of Marchantia is a gametophyte and is dioecious because the male and female reproductive structures are present on separate thalli or plant bodies. Thus, Marchantia is considered to be heterothallic.
The female thallus contains an archegoniophore which is the structure bearing the archegonium or female reproductive organ.
The male thallus bears an antheridiophore that is the structure bearing the male reproductive organ or antheridium.

Final answer
(d) Dioecious