The answer is B. Structural unemployment
People unemployed for longer periods are under the category of Structural unemployment, which is caused when the unemployed's skills don't correspond to the available jobs. This happens by changes in economy, in cases of a worker finding a job in a new industry, for example, with different skills requirements.
A. Frictional unemployment is the time the unemployed searches for a new job.
C. Seasonal unemployment occurs at certain times of the year.
D. Cyclical unemployment happens when production is low, because consumers are buying less.