C, D and E can be easily eliminated and we remain to chose between A and B.

A. Since 1960 the teenage unemployment rate has risen when the minimum wage has risen.
-- This will infact strengthen the argument. There might be possibility that if the minimum wage kept constant, unemployment can be reduced. Hence A is not a correct option.

B. Since 1960 the teenage unemployment rate has risen even when the minimum wage remained constant.
-- This is a correct answer choice. If this is true, it shows the minimum wage has no direct correlation with the unemployment. Hence even if the minimum wage is reduced to new lower lever, it is not necessary that unemployment will stop increasing.

C. Employers often hire extra help during holiday and warm weather seasons.
-- This is irrelevant information and has neutral effect on the argument. Hence C is a incorrect answer choice.

D. The teenage unemployment rate rose more quickly in the 1970's than it did in the 1960's.
-- This might be possible but has no impact on the conclusion. Hence D is an incorrect answer choice.

E. The teenage unemployment rate has occasionally declined in the years since 1960.
-- This is an opposite answer. It is strengthening the argument. Hence E cannot be a correct answer choice.