The correct answer is: B. Inelastic.
Demand for labor is inelastic when a small change in the price of labor leads to a small change in the quantity of labor demanded. This means that employers are not very sensitive to changes in the price of labor, and they will continue to demand the same amount of labor even if the price of labor goes up. This gives union leaders a stronger bargaining position, because they know that employers are not likely to hire fewer workers if they demand higher wages.
Option A, elastic, is the opposite of inelastic. When demand for labor is elastic, a small change in the price of labor leads to a large change in the quantity of labor demanded. This means that employers are very sensitive to changes in the price of labor, and they will hire fewer workers if the price of labor goes up. This gives union leaders a weaker bargaining position, because they know that employers are likely to hire fewer workers if they demand higher wages.
Option C, very large, is not a relevant factor in determining whether union leaders are in a better position to bargain for higher wages. The size of the labor force does not affect the elasticity of demand for labor.
Option D, permanent, is not a relevant factor in determining whether union leaders are in a better position to bargain for higher wages. The permanence of employment does not affect the elasticity of demand for labor.