SOLUTION:-
\[\left( D \right)\text{ }12\]
As indicated by the inquiry,
A line fragment \[AB\]in the proportion \[5:7\]
In this way, \[A:B\text{ }=\text{ }5:7\]
Presently,
Draw a beam \[AX\]making an intense point\[\angle BAX\],
Imprint \[A+B\]focuses at equivalent distance.
In this way, we have \[A=5\text{ }and\text{ }B=7\]
Thus, least number of these focuses \[=\text{ }A+B\text{ }=\text{ }5+7\text{ }=12\]