Explanation:

This is an example of 'गुण संधि' (Guna Sandhi) in Hindi grammar. According to the rules of Guna Sandhi, when the vowel 'अ' (a) or 'आ' (aa) is followed by 'इ' (i) or 'ई' (ee), they combine to form 'ए' (e). In 'स्व+इच्छा', 'स्व' (Swa) ends with 'अ' (a) and 'इच्छा' (Ichha) begins with 'इ' (i). Therefore, 'अ' + 'इ' = 'ए', resulting in 'स्वेच्छा' (Swechha). Option (1) 'स्विच्छा' incorrectly retains 'इ'. Option (3) 'स्वेक्षा' has incorrect spelling. Option (4) 'स्वैच्छा' would be formed by 'वृद्धि संधि' (Vriddhi Sandhi) where 'अ' + 'ए' = 'ऐ', which is not applicable here.