The correct answer is A. 0.67 L.
The effective length of a column is the length that is considered to be under axial load when calculating the column’s buckling strength. The effective length is always less than the actual length of the column, and it depends on the end conditions of the column.
A column that is effectively held in position at both ends and restrained in direction at one end is said to have fixed-fixed end conditions. The effective length of a column with fixed-fixed end conditions is 0.67 L, where L is the actual length of the column.
The other options are incorrect because they do not represent the effective length of a column with fixed-fixed end conditions. Option B, 0.8 L, is the effective length of a column with pinned-pinned end conditions. Option C, L, is the effective length of a column with pinned-fixed end conditions. Option D, 1.5 L, is the effective length of a column with fixed-pinned end conditions.