In average condition the coin is worth $15-20.
1921 was the last year for Morgans, so 1922 is a Peace dollar. It's worth about $27.
Mint state Morgans dated 1921 issued from the Denver Mint are very common with retail prices of $30.00-40.00
The US did not mint a dollar coin in 1968, therefore you don't have a 1968-D silver dollar.
A 1978-D Eisenhower dollar has no silver in it and is only face value.
Of all the Morgan dollars, the 1921 coins are the highest mintage most common. Regardless of mintmarks, values for circulated coins are $31.00-$40.00.
Sorry no O mint in 1921,it's a D or S. The 1921 date is so common even in mint state the buy value is below $40.00.
These coins contain no silver and are worth only face value.
PCGS shows a retail price of about $1300, although that may be a bit on the high side.
Please check your coin again. 1921-dated Peace dollars were only struck at Philadelphia. If your coin has a "D" above the "DO" in "DOLLAR" and the eagle's wings are outstretched, you have a Morgan dollar rather than a Peace dollar. There's more information at the question "What is the value of a 1921 Morgan silver dollar?"
In 1921 two different dollars were made both dated 1921. The most common is a Morgan dollar, some do have D or S mintmarks on the back of the coin just above the word DOLLAR. The other coin is a PEACE dollar that has this word on the back of the coin and will NOT have any mintmarks.
About $10.50.
There is no "series D silver dollar" (a coin) so I assume you're referring to a silver certificate. See the Related Question for more information.