Which of the following options is an example of a true solution of solid in solid?

A true solution is a homogeneous mixture of two or more chemically non-reacting substances whose composition can be varied within limits. A mixture is said to be a true solution if it satisfies the following two conditions: (i) The components of the mixture should be non–reacting. (ii) The mixture should be homogeneous. Steel is an alloy (homogeneous mixture of two or more metals) of iron 99.95% and carbon 0.05%. Thus, it is a true solution of solid and solid. Iron is not a mixture. It is an element made up of only one type of atom. Butter and gemstones are colloids (heterogeneous mixtures in which particle size is 1-100 nm).