Something to remember is how to multiply terms with same bases, but different exponents:
[tex] a^b \cdot a^c = a^{(b+c)} [/tex]
Essentially, we add the exponents.
Using the information above, we can find our answer:
[tex] 5^4 \cdot 5^8 = 5^{(4+8)} = \boxed{5^{12}} [/tex]
Our answer is [tex] \boxed{5^{12}} [/tex], or A.
