Comprehensive and Detailed Explanation:

Gross Pay Calculation:

Hourly Rate × Hours Worked = $10.00 × 40 = $400.00

Taxable Wages:

Gross Pay: $400.00

Pre-Tax Deductions: $50 (Medical) + $10 (Dental) + $25 (401k) = $85.00

Taxable Wages = $400 - $85 = $315.00

Federal Income Tax Withholding (Wage Bracket Method):

Based on IRS tables for single filers, FIT = approx. $20.00

Social Security & Medicare Taxes:

Social Security (6.2%) = $400 × 6.2% = $24.80

Medicare (1.45%) = $400 × 1.45% = $5.80

Total Deductions:

$20.00 (FIT) + $24.80 (SS) + $5.80 (Medicare) + $25.00 (Union Dues) = $75.60

Net Pay:

$400 - $75.60 = $259.99

Thus, the correct answer isD. $259.99.