The only correct answer is C.
A allows decimal and non-decimal values.
B allows any string and you could put a decimal point value into it.
C only allows whole numbers.
D is the same as B just a bit more redundant.
You can place an insertion point by clicking in the field or by clicking F2 keyboard shortcut. Insertion point is usually characterized by a blinking vertical line that allows you to insert a next character that you wanted.