Ruler Use for Straight Edges

The governing concept is using a measuring and guiding tool for accurate straight edges. A ruler can help draw a light guideline or mark the boundary before poster colour is applied, so option C is correct. Cotton may dab paint, clay is not a practical drawing guide, and a sponge is mainly useful for texture or broad application. The ruler should be used lightly and carefully so the guideline does not show through the finished colour.

The governing concept is using a straightedge as a guide for accurate linear boundaries. A ruler can be placed carefully along the intended edge so that the brush follows a straight path, making option A correct. Cotton and a sponge are useful for dabbing or soft effects, while clay cannot provide a practical, clean guide for this painting task. The ruler should be used without pressing wet paint beneath it.

A ruler has a straight edge that helps in drawing clean, straight and accurate lines or edges. Therefore, the ruler is correct. A brush is useful for applying colour, but it does not ensure a straight edge. Exam tip: Choose a ruler or scale when the question asks about making straight lines or edges.