In Nelson’s cell, Cathode is:
(a) graphite
(b) copper tube
(c) porous steel vessel
(d) iron task

Hint: Nelson’s cell is a type of electrolytic cell which is used to produce electrical energy from chemical energy. It consists of a special type of cathode where reaction takes place. It is used to produce Sodium Hydroxide solution.

Complete step by step answer:
(1) In Nelson’s cell, cathode is connected to the negative terminal of the battery, and anode is connected to the positive terminal of the battery. In between these two there is an electrolyte which is brine solution. Brine solution is $20\% $ ${\text{NaCl}}$ solution in water.
(2) Electrolysis process is carried out to produce the required Sodium Hydroxide $\left( {{\text{NaOH}}} \right)$ solution. First, the electrolyte is ionised by the following reactions:
$NaCl + {H_2}O \to N{a^ + }({\text{aq) + C}}{{\text{l}}^ - }({\text{aq)}} + {H^ + }({\text{aq)}} + O{H^ - }({\text{aq)}}$
(3) The oxidation occurs at anode. The reaction occurs is as follows:
$2C{l^ - }({\text{aq)}} \to C{l_2} + 2{e^ - }$
The reduction occurs at cathode. The following reaction occurs here:
$2{H^ + }({\text{aq)}} + 2{e^ - } \to {H_2}({\text{g)}}$
(4) We can clearly see that $N{a^ + }({\text{aq)}}$ is not involved in the electrode reaction at cathode, rather ${H^ + }({\text{aq)}}$ is involved. The free $N{a^ + }({\text{aq)}}$, and $O{H^ - }({\text{aq)}}$ ions which are present in the solution do not take part in the electrode reactions, combine to form Sodium Hydroxide $\left( {{\text{NaOH}}} \right)$.
(5) That’s why we use perforated or porous steel tank like vessel as cathode, so that, the Sodium Hydroxide $\left( {{\text{NaOH}}} \right)$ solution can be collected drop-wise in the catch basin. Porous steel vessel has many pores or small openings which allow the Sodium Hydroxide $\left( {{\text{NaOH}}} \right)$ solution to pass through them.

Hence, option (c) porous steel vessel is the correct answer.

Note:
Nelson's cell can have problems in which at anode, $C{l_2}$ can react with $O{H^ - }$ ion, and form Hypochlorite which are contaminants. This can be avoided by using asbestos diaphragm which keeps the two solutions separated.