Note that the option that you've highlighted is already deeply ITM. That's what justifies the massive bid premium of $2.50/shr.
The options with a lilac background are all in-the-money, meaning that if the underlying trades at its current value at expiration, all of those options will be exercised (in general*).
Can the PUT buyer exercise the option anytime they want; or,
Yes. (American options only.)
Only when the stock price hits the strike price on or before the expiry date?
A put doesn't even need to be in the money; they could theoretically exercise their right even if the option is OTM. This would be rare, though, because not only would they be relinquishing any extrinsic value in exercising the option early, but they'd also be selling or "putting" the shares to an assignee at a price that's lower than market value.
You generally as a long option holder want to sell shares at a price higher than what you paid for them. In the case of a put, that would mean that the fair market price per share must be below the option's strike price K.
'* an option owner can choose to not exercise an option if they so choose, but would need to inform their broker, who would then in turn tell the OCC not to assign