Which of the following is not a modern computer ? Tablet, Laptop, Personal Computer, Calculator

As Rooster said, the calculator is the oldest of the list you mentioned, and I don't think anyone would describe it as "modern" any more.

In comparison, "tablets" are the most modern device you mentioned. Even though they've been around since the early 1900s in one form or another, they did't really become mass-market consumer products until Apple launched the iPad in 2010.

And even that seems like a long time ago now!