Cumene process is the most important commercial method for the manufacture of phenol.
Cumene is:

The correct option is A (1-methylethyl) benzene
Cumene (isopropylbenzene) or (1-methylethyl) benzene is an organic compound obtained by Friedel-Crafts alkylation of benzene with propylene.
Cumene is oxidised in the presence of air to cumene hydroperoxide.
Now, cumene hydroperoxide is treated with dilute acid to prepare phenol and acetone as by-products. Acetone can be produce in large quantities by this method.

Hence 'a' is correct option.