Which one of them is incorrect

a If the current spot price of a non-dividend paying stock is $25, the call premium for that stock cannot be $30. 

b.

For an options holder, when a company participates in issuing stock dividends, both the strike price and the number of shares per option is increased by the size of the stock dividend.

c.

If you are a speculator and do not intend to hold the underlying stock at all, it may be beneficial to exercise an American option on a non-dividend paying stock early.

d.

The value of American options is decreasing in the time to maturity because there is less time for the stock price to reach a level favourable for the option to become in the money.