Option A: one.
In a number, to determine the number of significant figures following 3 rules are used:
(1) The digits which are non-zero are always significant.
(2) Any zeros between two digits which are non-zero are significant.
(3) In the decimal portion, trailing zeros are significant.
The number is 1 carbon atom, it has one non-zero digit.
Therefore, according to rules, there is only 1 significant figure (underlined): 1.
