Final answer:
Mendelssohn frequently employed chromatic modulations to distant keys, which was unusual for the time and contributed to the distinctive expressiveness of his music.
Step-by-step explanation:
The type of modulations that Felix Mendelssohn unusually used in his music is a) Chromatic modulations to distant keys. Mendelssohn was known for his creative use of harmony and modulation. He frequently made use of chromaticism, which allowed him to modulate to distant keys that were not closely related to the home key. This was quite distinct from the more common practice among other composers of his time, who typically used tonal modulations to move between keys that were closely related. Mendelssohn's use of chromatic modulations contributed to the expressive and colorful nature of his compositions, including works like his famous 'A Midsummer Night's Dream' Overture.