Question

Lymph vessels are united to form
(a) Lymph heart
(b) Cisterna chyli
(c) Thoracic duct
(d) Jugular vein

Answer 1

They are the largest lymphatic vessel in the human body. They are typically 38–45 cm in length and have an average diameter of about 5 mm. The vessel usually starts from the extent of the twelfth thoracic vertebrae (T12) and extends to the basis of the neck.

Complete answer:
Lymph vessels are united to form the thoracic duct. The lymph vessel carries chyle, a liquid containing both lymph and emulsified fats, instead of pure lymph. It further collects most of the lymph inside the body apart from the right thorax, arm, head, and neck (which are drained through the right lymphatic duct). It flows into the systemic (blood) circulation at the joining of the left subclavian and internal jugular veins, at the initiation of the brachiocephalic vein.
The lymph transport, in the thoracic duct, is principally caused through the action of breathing, supported by the duct's smooth muscle, and by internal valves which restrict the lymph from flowing back down again. There also are two valves at the junction of the duct with the left subclavian vein, to stop the flow of blood into the duct. In adults, the lymph vessel transports up to 4 L of lymph per day.

Additional information:
A lymph heart is an organ that pumps lymph in lungfishes, amphibians, reptiles, and flightless birds back to the cardiovascular system.
The cisterna chyli may be a dilated sac at the lower end of the lymph vessel in most mammals into which lymph from the intestinal trunk and two lumbar lymphatic trunks flow.
The jugular veins are veins that carry deoxygenated blood from the head back to the heart through the superior vein. It drains the brain.

So, the correct answer is ‘Thoracic duct’.

Note:
Thoracic duct is additionally understood as the left lymphatic duct, alimentary duct, chyliferous duct, and Van Hoorne's canal. When the duct ruptures, the resulting flood of liquid into the pleural cavity is understood as chylothorax.