The reason that I was late is I missed the bus.

[Harry12345]

Teahchers, Is the word that correct in this sentence? The reason that I was late is I missed the bus.

 

[Raymott]

Tea[STRIKE]h[/STRIKE]chers, Is the word "that" correct in this sentence? "The reason that I was late is I missed the bus."

Yes.
"The reason [that] I was late is [that] I missed the bus." Both are optional, so there's 4 possible choices of sentence.

 

[emsr2d2]

Unless this is some kind of exercise where your sentence has to start with "The reason ...", you could make life a lot simpler for yourself by saying "I was late because I missed the bus".