1. **Problem:** Find the factor of $(a + 5)(a - 9) - 15$ from the given options. 2. **Formula and rules:** Expand the product and simplify the expression. 3. **Work:** $$(a + 5)(a - 9) - 15 = a^2 - 9a + 5a - 45 - 15 = a^2 - 4a - 60$$ 4. Factorize $a^2 - 4a - 60$: Find two numbers whose product is $-60$ and sum is $-4$. These are $-10$ and $6$. $$a^2 - 4a - 60 = (a - 10)(a + 6)$$ 5. **Answer:** Among the options, $a - 10$ matches one factor. Final answer: খ) $a - 10$