Question

a b 15 12 9 c e 3 d 5 f 4 not similar or not necessarily similar similar: △abc ~ △ by the select

Explanation:

Step1: Find ratios of sides

For $\triangle ABC$: sides are $15, 12, 9$. For $\triangle DEF$: sides are $5, 3, 4$. Calculate ratios: $\frac{15}{5}=3$, $\frac{12}{4}=3$, $\frac{9}{3}=3$. All ratios equal.

Step2: Determine similarity criterion

Since corresponding sides are proportional (ratio $3$), use SSS (Side - Side - Side) similarity. So $\triangle ABC \sim \triangle EFD$ (or $\triangle DFE$ etc., but matching sides: $AB = 15$ (matches $EF = 5$ scaled), $AC = 12$ (matches $DF = 4$ scaled), $BC = 9$ (matches $DE = 3$ scaled), so $\triangle ABC \sim \triangle EFD$ by SSS similarity.

Answer:

Similar: $\triangle ABC \sim \triangle EFD$ by the SSS (Side - Side - Side) Similarity Criterion