Yes, he can be. However, it is not clear that he has to reply to it. He can refuse most subpoena because of his right of executive privilege.
Theodore Roosevelt
Andrew Johnson
Woodrow Wilson (1919)
No, a sitting president cannot be charged with treason.
Susan faced a dilemma when she was subpoenaed by the courts
He was the sitting Governor of the state of Texas.
James A. Garfield
In March of 1807, President Jefferson issued a written statement roundly condemning the institution of slavery in the US. He was the first sitting president to do so before President Abraham Lincoln's time.
The past tense of "subpoena" is "subpoenaed."
No US president- elect has been assassinated. Four sitting presidents have been assassinated: Lincoln.Garfield,McKinley and Kennedy.
a president that is in ofice
Sitting means "current" or "active" president. According to the smarties on StackExchange politics, it is acceptable, to call ex-presidents "Mister [lastname]", or "Former President", or even "Mister President" - even though they no longer have that role. Famously, Marilyn Monroe addressed a sitting member of the executive staff (President Kennedy) as "Mister President" in a happy birthday song, and most guys who listened to that.. well, their members were standing rather than sitting afterwards. ;)