Shrinking of nucleus is caused when this inactivates

The correct option is lamin.

Nuclear lamins are fibrous proteins that provide structural support and regulate key activities in the cell nucleus. They polymerize to form a dense, scaffold-like network known as the nuclear lamina, which is positioned on the inner surface of the inner nuclear membrane. This lamina is essential for maintaining the shape, mechanical stability, and overall structural integrity of the nucleus.

During programmed cell death (apoptosis), a cascade of specialized proteolytic enzymes called caspases is activated. Caspases target and cleave nuclear lamins, resulting in their inactivation and disassembly. Once this structural support system is dismantled, the nuclear envelope becomes unstable, leading to chromatin condensation and the characteristic shrinking (known as pyknosis) and fragmentation of the cell nucleus.

Here is a brief look at why the other options are not responsible for this nuclear change:
1. Gelsolin: This is an actin-regulating protein found mainly in the cytoplasm that breaks down actin filaments, rather than forming the structural envelope of the nucleus.
2. Tubulin: These proteins form microtubules, which make up the eukaryotic cytoskeleton and the mitotic spindle during cell division, but do not structure the nuclear envelope.
3. Actin: This protein forms microfilaments that are crucial for cellular motility, contraction, and maintenance of overall cell shape in the cytoplasm.