Step 1: Concept
Family of planes: $P_1 + \lambda P_2 = 0$.
Step 2: Analysis
$(x+2y-z+1) + \lambda(3x-y-4z+3) = 0$.
Passes through $(1, 1, 1)$: $(1+2-1+1) + \lambda(3-1-4+3) = 0 \implies 3 + \lambda(1) = 0 \implies \lambda = -3$.
Step 3: Calculation
Substitute $\lambda = -3$: $(x+2y-z+1) - 3(3x-y-4z+3) = 0$
$x+2y-z+1 - 9x+3y+12z-9 = 0 \implies -8x + 5y + 11z - 8 = 0$.
Multiply by -1: $8x - 5y - 11z + 8 = 0$.
Step 4: Conclusion
Hence, the correct equation is $8x - 5y - 11z + 8 = 0$.
Final Answer: (C)