Final answer:
Franz Liszt is credited with inventing the symphonic poem in 1848 and with it the concept of thematic transformation.
Step-by-step explanation:
The inventor of the symphonic poem in 1848, along with the concept of thematic transformation, is Franz Liszt. This style reflects the Romantic era's emphasis on the expression of human emotion and innovation in musical forms. Liszt's developments allowed for the integration of poetic and narrative elements into instrumental music, which was a significant departure from the Classical tradition that emphasized structured forms such as the symphony and the concerto. Instead of strictly adhering to established forms, Liszt's symphonic poems were single-movement orchestral works that used thematic transformation to develop musical motifs within a programmatic framework.