I'm going with pleading the Fifth, despite his lawyer's statement the other day...
FBI Agent Who Sent Anti-Trump Texts Will Testify If Asked
Strzok, who was singled out in a Justice Department inspector general report for the politically charged messages, would be willing to testify without immunity, and he would not invoke his Fifth Amendment rights, said his attorney, Aitan Goelman.
My reasoning - he thought he could stall testifying...
https://www.zerohedge.com/news/2018-06-22/strzok-subpoenaed-must-appear-congress-five-days
Strzok agreed to testify earlier, however Grassley issued the subpoena to appear on June 27 at 10:00 a.m. after the FBI agent would not commit to a date.
He wasn't asked, a subpoena compels his appearance.
“FBI Agent Who Sent Anti-Trump Texts Will Testify If Asked”
My take, FWIW, he wasn’t “asked”, he was subpoenaed. Therefore, he will plead the fifth.