A mixture is a combination of two or more pure substances in which each pure substance retains its individual chemical properties.

A homogeneous mixture is a solid, liquid, or gaseous mixture that has the same proportions of its components throughout any given sample. Conversely, a heterogeneous mixture has components whose proportions vary throughout the sample.

Options (a) and (b) are correctly labelled. But, options (c) and (d) are wrong or incorrectly labeled. Let's see the reason for that. Oxygen and nitrogen will form a homogeneous mixture in a scuba tank because gases mix uniformly in any proportion to form a homogeneous mixture. Also, aqueous solution of alcohol is a homogeneous mixture, because the two liquids are miscible, soluble in all proportions.