What is a better way to create a darker shade of the same acrylic hue?

Correct answer: Adding a small amount of a suitable darker analogous or complementary colour

The governing concept is controlled colour mixing. To darken a hue while preserving its character, begin with a very small amount of a suitable darker analogous colour; a carefully chosen complementary can also reduce lightness, though too much may neutralise the mixture. Option B is correct. White raises value and makes a tint, varnish changes sheen, and soap is not an appropriate paint additive. Mixing gradually allows accurate adjustment.