$(a)$ $PhCOOH$
$(B)$ $o-NO_2C_6H_4COOH$
$(C)$ $p-NO_2C_6H_4COOH$
$(D)$ $m-NO_2C_6H_4COOH$
$(A)\,{{C}_{8}}{{H}_{10}}\xrightarrow{KMn{{O}_{4}}}(B){{C}_{8}}{{H}_{6}}{{O}_{4}}\xrightarrow[Fe]{B{{r}_{2}}}{{C}_{8}}{{H}_{5}}Br{{O}_{4}}(C)$ (માત્ર એક જ નીપજ )