Answer: $\mathbf D$
Explanation:
$\mathrm y = \log_{10}3245.7 \\= \log_{10}3.2457\times 10^3 \\= \log_{10}3.2457+\log10^3 \\= \underbrace{\log_{10}3.2457}_\text{less than 1} + \underbrace{3\log_{10}{10}}_\text{=3} \\= 3+ \text{ Some decimal values} = 3$
Since, the values for greatest integer function for $3.2 = 3$ or gif for $3.7 = 3$, i.e., it always gives the greatest integer value.
So, the answer comes out to be $3$
$\therefore \mathbf D$ is the correct option.