Explanation:

- First, we identify the factors:

- Factors of 108: $2^2\times 3^3\times 6$

- Factors of 72: $2^3\times 3^2$

- Factor of $5a$ includes 'a'.

- The highest common factor (HCF) of 108, 72, and 5a is given as $a$. Thus, $a$ itself is part of the common divisors.

- When $a$ is the HCF, $a=36$ as 36 is the highest common factor of 108 and 72. Therefore, by inference $a$ does not divide 5 completely.

- The Least Common Multiple (LCM) is the smallest number that is divisible by each of given numbers.

- Calculate LCM:

- LCM of 108 and 72 can be calculated as 432.

- For 108, 72, and $a=36$: LCM will include most abundant powers of composite numbers from above prime factorization.

- Check Options:

1. Option 432: LCM of 108, 72, and 36 is possible as it accounts for all factors.

2. Option 324: Not directly divisible by 72.

3. Option 108: Smaller than initial 108.

4. Option 216: Not consistent with the LCM calculation.

- Correct Choice is:

Option 1: 432