Am I a Canadian Citizen by Descent?

[hawk39]

So yes there are technically several options when he could have inherited that citizenship.

Technically yes, there are several options; however they are based on suppositions that the OP did not confirm. The responses from the other posters are based on the information that the OP gave, simply that his/her grandmother was born in Canada. Based on this information, your introduction of "compulsory registration of the 80ties" was not needed based on the explanation I wrote in my initial post.

1. What was the citizenship of father of that child at the time of birth. Especially if his grandfather was British at the time his parent was born. (because all she needed not to loose Canadian citizenship in 1947 was not to loose British before, so marrying a British subject abroad would have been enough).

This "grandfather" would also have needed to become a Canadian citizen under the 1947 Act, either having been born in Canada and not losing his British subject status, or as a British subject that held permanent residence in Canada for five years prior to commencement of the 1947 Act. The term "British subject" refers to all people with said status within all of the British dominions, not just Canada, so "marrying a British subject abroad" is inaccurate.

2. Was that child born out of wedlock or not. Because if not that child would become Canadian by descend by 1947.

Under British Nationality Law at that time, I can't find anything under 1(1)(b) regarding wedlock, so I believe wedlock is not a factor. The "child" (i.e. the OP's parent) was also born in the US (which the OP already stated) and is a US citizen, and would not have become a Canadian citizen under 4(b) of the 1947 Act as an alien (i.e. citizen of another country) prior to the commencement of the Act.

3. Was that child his mother or his father. And if it was mother, what was the status of father (especially regarding possible dual citizenships (like British for example).

The OP already stated that both of his/her parents were born in the US. Dual citizenship was not allowed until 1977 so this I believe this point is moot.