The correct answer is block.
A block of flats is a building that contains multiple flats. Flats are also known as apartments, condominiums, or tenements. They are typically smaller than houses and are often located in urban areas. Flats can be rented or owned.
A pack is a group of things that are packed together. For example, a pack of cards or a pack of batteries.
A set is a group of things that are related to each other. For example, a set of tools or a set of dishes.
A conclave is a meeting of people who are working on a secret project. For example, a conclave of scientists or a conclave of politicians.
A suite is a group of rooms that are connected to each other. For example, a hotel suite or a hospital suite.
In the context of the question, “I want to have a . . . . . . . . of flats on rental basis,” the most likely answer is “block.” This is because a block of flats is a building that contains multiple flats, which is what the person is asking for. The other options are not as relevant to the question.