In the given question, a statement of Assertion is followed by a statement of Reason. Mark the correct answer.

Assertion: The figures for mortality and disease were difficult to collect for the census officers in the 19th century,
Reason:  All the deaths were not registered, and illness was not always reported, nor treated by licensed doctors.

Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Often people themselves refused to cooperate or gave evasive answers to the census officials. For a long while they were suspicious of census operations and believed that enquiries were being conducted to impose new taxes. Upper-caste people were also unwilling to give any information regarding the women of their household: women were supposed to be secluded within the interior of the household and not subjected to public gaze or public enquiry.
Census officials also found that people were claiming identities that they associated with higher status.
Similarly, the figures for mortality and disease were difficult to collect, for all deaths were not registered, and illness was not always reported, nor treated by licensed doctors.
Thus historians have to use sources like the census with great caution, keeping in mind their possible biases, recalculating figures and understanding what the figures do not tell.