ANS:C - four times

Columns may be made of plain concrete if their unsupported lengths do not exceed their least lateral dimension four times. This means that if the unsupported length of a column (i.e., the distance between its supports) does not exceed four times its least lateral dimension (the smallest dimension of the column cross-section), it can be constructed using plain concrete without requiring additional reinforcement. However, it's important to note that this is a general guideline, and specific design requirements may vary based on factors such as structural loads, building codes, and safety considerations. In practice, engineers evaluate various parameters including the column's slenderness ratio, axial load, and lateral support conditions to determine the appropriate construction material and reinforcement requirements for columns.