Correct option is (d)
The velocity of conduction of nerve impulse in frog is 30 metres per second and that of mammal is 120 metres per second. The threshold value of any nerve fibre is the minimum strength of stimulus which initiates action potential in that nerve fibre.

Option b is correct, Nerve impulse are conducted with great velocity in mammalian nerve,the speed of conduction may be as high as 100 meters per second.

The correct answer is option 'B': (a) two Na ions are exchanged and (b) three K ions are exchanged when the sodium and potassium pump is activated.

The sodium-potassium pump is a crucial membrane protein responsible for maintaining the concentration gradients of sodium and potassium ions across the cell membrane. It plays a vital role in various cellular processes, including nerve cell signaling, muscle contraction, and maintaining osmotic balance.

The process of the sodium-potassium pump can be divided into several steps:

1. Binding of ATP: The pump binds to adenosine triphosphate (ATP), which provides the energy required for the pump's activity.

2. Sodium (Na) ion binding: The pump has high affinity for sodium ions. Three sodium ions from the intracellular fluid bind to the pump.

3. Phosphorylation: ATP is hydrolyzed, and a phosphate group is transferred to the pump. This phosphorylation step causes a conformational change in the pump, leading to the release of the bound sodium ions to the extracellular fluid.

4. Potassium (K) ion binding: The conformational change exposes the binding sites for potassium ions. Two potassium ions from the extracellular fluid bind to the pump.

5. Dephosphorylation: The pump undergoes dephosphorylation, resulting in another conformational change. This change allows the release of the bound potassium ions into the intracellular fluid.

6. Resetting: The pump returns to its original conformation, ready to repeat the process.

The sodium-potassium pump exchanges three sodium ions for every two potassium ions. This exchange is crucial for maintaining the concentration gradients of these ions. The pump helps to keep the intracellular sodium concentration low and the intracellular potassium concentration high, which is important for various cellular functions.

In conclusion, when the sodium and potassium pump is activated, two sodium ions are exchanged for every three potassium ions. This exchange is necessary for maintaining the proper concentration gradients across the cell membrane, which is vital for the proper functioning of cells.