The critical stress on a column for elastic buckling given by Euler's formula, is

• A. <img src="/_files/images/civil-engineering/steel-structure-design/96-14-57-1.png">
• B. <img src="/_files/images/civil-engineering/steel-structure-design/96-14-57-2.png">
• C. <img src="/_files/images/civil-engineering/steel-structure-design/96-14-57-3.png">
• D. <img src="/_files/images/civil-engineering/steel-structure-design/96-14-57-4.png">