Taj Mahal is said to be suffering from 'Marble Cancer'. What is Marble Cancer?

Question

$\begin{array}{1 1} (a)\; \text{acid rain which corrodes marble} \\ (b)\; \text{fungus formed at Taj Mahal} \\ (c)\; \text{smoke filling the Taj Mahal from adjoining industries } \\ (d)\; \text{yellowing of marble on account of soot particles } \end{array}$