Question on selling PUT option

That put is in-the-money so the only reason it wouldn't be exercised is if the price of the underlying goes above (or very near to) 7.5 by expiration.

Most options can be exercised at any point in time but there are some underlying that use European-style options which cannot be exercised early (not applicable for this one because it's already 0 dte and it's not an underlying with European-style options).

Selling a put gives you the obligation to buy 100 shares at the strike price if assigned. If you sell the option for 3.2 you would need the stock price to be above 4.3 to break even (because you'll be paying 7.5/share to buy 100 shares). With so little time to expiration and being relatively deep in-the-money the option value will be almost all intrinsic and you'll see it has poor liquidity. Not many compelling reasons to sell that put, after fees you'd likely be better off just buying 100 shares at current price if you are interested in it.

European options (most indexes) cannot be exercised until expiration. American style options (equities) can be exercised at any time.

There is no incentive for someone to exercise an in-the-money (ITM) option early if there is time premium remaining because doing so would throw away the time premium. The exception to this would be if there is a pending dividend and the amount of the time premium is less than the dividend.

With KRMD at $4.40, it doesn't make a lot of sense to sell the Jun 18th $7.50 put for $3.20 because there's only 10 cents of time premium. And that assumes someone would do so when the quote is $2.50 by $3.60 and that's not likely.

The intrinsic value of this put is $3.10 so anyone selling the put for less than the intrinsic value would provide the buyer with the opportunity to do a discount arbitrage which would involve exercising that long put. That could lead to you be assigned early (if you had sold an expiration further out than tomorrow).

Last of all, ITM options are automatically exercised by the Option Clearing Corp at expiration. The owner of the option can inform the OCC via the broker not to automatically exercise the option. The seller of an option has no such choice.

IMPORTANT:

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*).

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Next...

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.

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'* 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