Answer
(a) element
(b) homogeneous mixture
(c) compound
(d) heterogeneous mixture
Work Step by Step
(a) element
Gold is an element because it cannot be broken down any further into any constituent parts that can still be identified as gold. On the periodic table, gold is element 79 with the elemental symbol Au.
(b) homogeneous mixture
Milk is a homogeneous mixture because it is made of a combination of different substances combined physically in a consistent manner throughout.
(c) compound
Sugar is a compound because it is made of a combination of different elements combined chemically in fixed ratios.
(d) heterogeneous mixture
A vinaigrette is a heterogeneous mixture because it is made of different substances, but you would have to mix it up to have a consistency that is more or less uniform. If left untouched, the vinaigrette will separate into its different constituent parts.