How many subnets can a single AWS VPC contain?

Correct answer: Unlimited (limited by AWS account limits)

Option D is correct because AWS does not impose a fixed upper limit on the number of subnets per VPC in the way older documentation implied; the practical limit is governed by adjustable AWS account-level service quotas (default 200 subnets per VPC, which can be increased), making 'unlimited subject to account limits' the most accurate characterization among the choices. Option A is incorrect because 64 is not a documented AWS limit for subnets per VPC. Option B is incorrect because 256 is not an accurate fixed limit for subnets per VPC. Option C is incorrect because 128 does not correspond to a documented AWS quota for subnets per VPC.