Here's why:
* Instrumental composition: It's a piece of music played by instruments.
* Several movements: It's divided into distinct sections.
* Based to some extent on literary or pictorial ideas: This indicates a program, a story or concept that the music is meant to evoke.
Examples:
* Berlioz's Symphonie Fantastique: This famous symphony is a prime example, telling the story of an artist's love, obsession, and eventual death.
* Tchaikovsky's Symphony No. 6 ("Pathétique"): While less explicitly programmatic, it evokes a mood of melancholy and tragedy.
While not all symphonies are programmatic, this particular combination of elements strongly suggests this type of composition.