Complete the frustrated student's text message to their friend by selecting the grammatically correct word.

The coffee shop is great, but the Wi-Fi ___ working today!

The correct answer is isn't.

"Wi-Fi" is a singular noun (an "it"), so we use the singular form of the verb to be (is). Combined with not, it becomes the contraction isn't.

We use to be here because it is part of the present continuous tense ("is working").