The correct answer is option 3 - Nucleoplasm. The nucleus of a cell contains a viscous liquid called nucleoplasm. Nucleoplasm is a gel-like substance that fills the space within the nucleus and surrounds the nucleolus and chromatin. It is a complex mixture of water, ions, enzymes, proteins, and other biomolecules.
Option 1 - Cytoplasm, refers to the fluid and organelles outside the nucleus but within the cell. It is not contained inside the nucleus.
Option 2 - Protoplasm, refers to the living matter of a cell, which includes both the cytoplasm and nucleus. While nucleoplasm is part of the protoplasm, it specifically refers to the liquid inside the nucleus.
Option 4 - Nucleosome, on the other hand, refers to a structural unit of DNA and histone proteins in the chromatin, which is found inside the nucleus but is not a liquid.
Therefore, the correct answer is option 3 - Nucleoplasm, as it is the specific liquid contained inside the nucleus of a cell.