What is the official letter for the bedrock soil horizon? I know that the letters to represent the A, B, and C horizons are widespread, but the fourth layer (bedrock) is referred to with different names. Some call it the D-Horizon, some the R-Horizon, and some just say bedrock. Is there any standard or official term, and if so, what is it?
It depends on which classification system you're using, which varies by country.
According to the USDA Soil Taxonomy, R is used to designate bedrock.
Other countries and classification systems may (or may not) use different letters. Check out the Compendium of On-Line Soil Survey Information for other classification systems.
To answer your second question, there are standards, depending on which classification system or country or even discipline you're in. Soil scientists in the US use the USDA soil taxonomy, or R to designate bedrock.