The Comparative Degree denotes the existence of a higher degree of the quality than the positive. It is used when two things (or two sets of things) are compared.
The format for comparative degree is '<adjective> + er than' or 'more/less <adjective> than'.
Hence option C is correct as more is the comparative form.
'Less' is the positive form while ' the most and the least' are superlative forms.
Hence, option A, option B and option D are incorrect.