Choose the correct phrase to complete the astronaut's space log.

After ________ delay, we are finally ready to launch this rocket to Mars!

The correct answer is three months'.

We use apostrophes in time expressions when the time period acts as a possessive adjective describing the noun (e.g., a delay of three months). Since "months" is a regular plural noun ending in s, we simply add an apostrophe to the end (months').