The pentagon.
A confederation
There are no square nickels. All US nickels are round, and Canadian nickels are either round or dodecagonal (12-sided). Please check your coin again and post a new, separate question with its date and country of origin.
A US "penny" is actually a 1 cent coin, worth 1/100 of a dollar. A British penny (the coin's official name) is worth 1/100 of a pound sterling.
The US 10 cent coin has president Roosevelt.
No circulating US coin has 11 sides.Early designs for the Susan B. Anthony dollar (1979-81 and 1999) had 11 sides. However the final design had a standard round edge with an 11-sided interior border. The US Mint felt it was impractical to strike a multisided coin despite the fact that other countries had been doing so for many years.
Nonagon is the name of a 9 sided shape. The US Steel Tower in Pittsburgh, Pennsylvania is a nonagon.
...Of what US coin? There have been several US coins intended for circulation ranging from half a cent to $20.
Nickel is an element that has the same name as a US coin worth five cents.
A quarter
the nickel.
dime
The nickel.
The US coin named after an element is the nickel. It is composed mainly of copper and nickel, hence the name.
The pentagon.
the biggest is the dollar coin and like its name it's worth a dollar
the first us coin was the pine tree shilling