Expression: It's about time

[Cruella]

Test No. incompl/advan-30 "Letter Writing (2)", question 1

Dear Charles I thought it was ......... time I wrote to you and told you what I've been up to.

(a) in
(b) about
(c) at
(d) by

Test No. incompl/advan-30 "Letter Writing (2)", answer 1

Dear Charles I thought it was about time I wrote to you and told you what I've been up to.

Correct answer: (b) about
_________________________

why?

Cruella

[Torsten]

It's about time is an expression that means that the time has arrived for something to happen.

Also, you should take a look at this post It's about time

TOEIC listening, photographs: On a laptop