The sudden loss of a would necessarily cause the loss of most or all of a food web.
a. producer
b. mutualistic species
c. keystone species
d. secondary consumer

The answer to the question stated above is letter a. producer.
The sudden loss of a producer would necessarily cause the loss of most or all of a food web.
Producers are the beginning of a simple food chain. Producers are plants which are at the beginning of every food chain that involves the Sun. All energy are coming  from the Sun and plants are the ones who make food with that energy, without producers the transfer of energy is destructed. 

I’m going to go with b!