During inspiration, which of the following condition takes place?

Thoracic volume increases.

The correct answer is Option (1) - Thoracic volume increases.

• The internal body cavity of mammals is divided by a muscular diaphragm into a thoracic or chest cavity and an abdominal cavity.
• The thoracic cavity is an air-tight cavity, which houses the lungs. The thoracic cavity being airtight follows Boyle's law. According to Boyle's law, pressure and volume are inversely related.
• During inspiration with the help of movements in the diaphragm and costal muscles between ribs, the volume of the thoracic cavity is increased, the pressure decreases as compared to atmospheric pressure and the air comes inside the lungs. During expiration, the volume of the thoracic cavity is decreased, pressure increases as compared to atmospheric pressure, and air moves out of the lungs.