Horses were injected with toxins of diseases until their blood built up immunities.
A. given immunity to diseases
$\rightarrow$ It is correct since they were given toxins of diseases to attain immunity to diseases.
B. Generally quite immune to diseases.
$\rightarrow$ We can't say horses are generally immune to diseases. (since they are given toxins to acheive immunity).
C. given medicines to fight toxins
$\rightarrow$ It is incorrect since they were given toxins of diseases not medicines to fight toxins.
Then a serum was made from their blood. Serums to fight with diphtheria and tetanus were developed this way.
D. given diphtheria and tetanus serums
$\rightarrow$ It is incorrect since the serum was made from their blood and not given to them.
$\therefore$ Option A is the right answer.