Considering only Market drops:
The product $HD$ has experienced a market drop In ${\text{Bombay}}$ $ \{(15 - 20) = -5\hspace{0.2cm} i.e \hspace{0.2cm}\dfrac{-5}{20}\times100 = \color{lightblue}{25\%}\} $, $\text{Calcutta}$ $ \{(30 - 35) = -5\hspace{0.2cm} i.e \hspace{0.2cm}\dfrac{-5}{35}\times100 = \color{lightblue}{14.28\%}\} $ & $\text{Delhi}$ $ \{(15 - 20) = -5\hspace{0.2cm} i.e \hspace{0.2cm}\dfrac{-5}{20}\times100 = \color{lightblue}{25\%}\}$.
The product $CO$ has experienced a market drop In $\text{Calcutta}$ $ \{(15 - 30) = -15\hspace{0.2cm} i.e \hspace{0.2cm}\dfrac{-15}{30}\times100 =\color{lightblue}{ 50\%}\} $ & $\text{Delhi}$ $ \{(10 - 15) = -5\hspace{0.2cm} i.e \hspace{0.2cm}\dfrac{-5}{15}\times100 = \color{lightblue}{33.33\%}\} $.
The product $BN$ has experienced a market drop In $\text{Bombay}$ $ \{(40 - 45) = -5\hspace{0.2cm} i.e \hspace{0.2cm}\dfrac{-5}{45}\times100 = \color{lightblue}{11.11\%}\} $ .
The product $MT$ has experienced a market drop In $\text{Madras}$ $ \{(45 - 40) = -5\hspace{0.2cm} i.e \hspace{0.2cm}\dfrac{-5}{40}\times100 = \color{lightblue}{12.5\%}\} $ .
So, $\color{green}{\text{the largest percentage drop in market shares was experienced by}}$ $\color{gold}{CO}$ $\color{green}{in}$ $\color{red}{\text{Calcutta}}$ $\color{green}{\text{which was}}$ $\color{blue}{\dfrac{15-30}{30} \times 100 = \dfrac{-1}{2}\times100 = 50\%}$