This is a Multiple Choice icon. We've made it in iOS style, first introduced in iOS version 7 and supported in all later releases up until now (at least iOS 11). This style is based on thin two-pixel lines and is optimized for 50x50 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 12,500 icons, all in the same style and quality.
The image is of three squares that are horizontally lined up. They are not touching each other. To the right of the squares are horizontal lines of the same length. The top and bottom squares are filled in. The middle square is blank.