What is the use of sampling ports?

The correct answer is Option (4) → To withdraw small volumes of the culture periodically

Option (1) To withdraw large volumes of culture periodically — Incorrect. Only small samples are taken, to check the culture without disturbing it.

Option (2) To add large volumes of raw culture periodically — Incorrect. Sampling ports take material out; they are not for adding culture.

Option (3) To add large volume of raw culture in the beginning — Incorrect. Again, a sampling port is for taking samples, not for filling the tank.

Option (4) To withdraw small volumes of the culture periodically — Correct. A bioreactor has sampling ports through which small volumes of the culture are taken out from time to time. This lets the conditions inside be checked while the process goes on.

NCERT Extract:
Obtaining the Foreign Gene Product – “… A stirred-tank reactor is usually cylindrical or with a curved base to facilitate the mixing of the reactor contents. The stirrer facilitates even mixing and oxygen availability throughout the bioreactor. Alternatively air can be bubbled through the reactor. If you look at the figure closely you will see that the bioreactor has an agitator system, an oxygen delivery system and a foam control system, a temperature control system, pH control system and sampling ports so that small volumes of the culture can be withdrawn periodically.”