The fluid filled space within the nuclear envelope

The fluid-filled space within the nuclear envelope is called the cytoplasm

The correct answer is nucleoplasm.

The nucleus of the majority of the cells comprises a substance, which suspends compositions inside the nuclear membrane. Similarly, to the cytoplasm, the nucleus comprises of nucleoplasm, also called karyoplasm.

The nucleoplasm is a kind of protoplasm, which comprises the majority of water, dissolved ions, and a mixture of different molecules. It is entirely enveloped inside the nuclear envelope or nuclear membrane. It is a sticky liquid, highly gelatinous, which supports the nucleoli and chromosomes.

The main activity of the nucleoplasm is to function as a suspension material for the organelles within the nucleus. It also assists in sustaining the composition and shape of the nucleus and plays an essential function in the transportation of substances, which are important to cell function and metabolism.