Here, the correct answer is Furniture. i.e.I have to buy a lot of furniture for my new apartment.
You might be amazed after knowing that there are some nouns in our grammar which have only singular form and some have only plural form. Furniture isa noun which has a singular form. There is noplural form of furniture.
The answer to this is furniture. Furniture is one of the nouns that only have a singular form. This means that even if you are trying to state that you have to purchase different items, you can only use the singular form of the noun. There are still other items that fall under this category such as luggage that people normally make a mistake with. The news is another example.
Even if you want to state that you have watched the news, this does not mean that you only watched one story. It may mean that you have watched the whole news show but because you are using the singular form of the noun, you still need to use news.