Let's go through the historical sound changes and match them to the correct processes:
1. P. rupa → rup:
This is an example of Apocope, which refers to the loss of one or more sounds from the end of a word. In this case, the final vowel 'a' is dropped from 'rupa', resulting in 'rup'. Therefore, the correct process is (2) Apocope.
2. Q. sapta → sa:ta:
This is an example of Compensatory lengthening, where a vowel is lengthened to compensate for the loss of another phoneme. The loss of 'p' in 'sapta' results in the lengthening of the 'a' sound, creating 'sa:ta'. Therefore, the correct process is (4) Compensatory lengthening.
3. R. bhodha → bodha:
This change involves the simplification or change in the articulation of a sound, where the 'bh' sound is replaced by 'b'. This is an example of Grassman's law, which involves the simplification of consonants in specific phonological environments. Therefore, the correct process is (1) Grassman's law.
4. S. səŋkələn → səŋkələn:
This is an example of Assimilation, where a sound changes to become more like a neighboring sound. In this case, the 'k' and 'l' sounds may influence each other, leading to assimilation. Therefore, the correct process is (3) Assimilation.
Thus, the correct answer is P-2, Q-4, R-1, S-3.