Given: (1 – x + x2)n = a0 + a1 x + a2 x2 + ....+a2n x2n ...(1)In order to find a0 + a2 + a4 +....+ a2n i.e. all even terms are involved ∴ replace x by 1 in equation (1) we get (1-1+1)n=a0+a1+a2+ .... +a2ni.e. 1=a0+a1+a2+ .... +a2n ....(2) and now replace x by –1 in equation (1), we get (1+1+1)n=a0-a1+a2-a3+ .... +a2ni.e. 3n=a0-a1+a2-a3+ .... +a2n ...(3)
By adding (2) and (3), we get 3n+1=2a0+2a2+2a4+....+2a2ni.e. a0+a2+a4+....+a2n=1+3n2 Hence, the correct answer is option A.