Final answer:
The action taken by the insurer to increase market share in the workers compensation insurance field in construction is known as market penetration or market expansion.
Step-by-step explanation:
The type of underwriting activity where an insurer's staff underwriter decides to increase its market share of workers compensation insurance in the construction industry is known as market penetration or market expansion. An insurer may try to achieve this by differentiating their product, adjusting prices, or enhancing distribution strategies to reach more of the construction industry. The goal is to grow the insurer's business within a specific market segment that has been identified as having growth potential, while managing risks related to adverse selection, where a higher proportion of high-risk individuals might seek insurance compared to those at lower risk.